Images incorporating microstructures

ABSTRACT

A method of generating an image incorporating a microstructure includes the steps of obtaining an original image, generating a microstructure, and rendering a region or the whole said original image with said microstructure. The operation of generating the microstructure includes an automatic synthesis of microstructure elements from original microstructure shapes.

BACKGROUND OF THE INVENTION

The present invention relates generally to images incorporatinginformation both at the global level and at the microstructure level andto a method of generating such images. The information at themicrostructure level offers, in particular, protection againstcounterfeiting and may be used as a security feature in documents. Theinvention also relates to documents comprising security features, and toa method of generating such documents, which may include for example,value bearing commercial instruments, certificates, coupons and personalidentification instruments.

The term ‘images’ used herein shall be understood in the broad sense tomean any visual representation of matter that may be printed ordisplayed on a display, for example text, pictures, photographs,drawings and so on.

A microstructure may comprise microstructure elements such as a text, alogo, an ornament, a symbol or any other microstructure shape. When seenfrom a certain distance, mainly the global image is visible. When seenfrom nearby, mainly the microstructure is visible. At intermediatedistances, both the microstructure and the global image are visible.

Several attempts have already been made in the prior art to generateimages incorporating information at the microstructure level, where fromfar away mainly the global image is visible and from nearby mainly themicrostructure is visible. A prior art method hereinafter called“Artistic Screening” was disclosed in U.S. Pat. No. 6,198,545 and in thearticle by V. Ostromoukhov, R. D. Hersch, “Artistic Screening”,Siggraph95, Computer Graphics Proceedings, Annual Conference Series,1995, pp. 219-228. This method requires however significant efforts bygraphic designers in order to create the microstructure and is limitedto bi-level images, i.e. images in black-white or a single color andwhite.

A prior art method for incorporating a microstructure into an image bycomputing color differences is disclosed in European Patent application99 114 740.6. This method does not modify the thickness of themicrostructure according to the local intensity of the image.

Another method hereinafter called “Multicolor Dithering” is disclosed inthe article by V. Ostromoukhov, R. D. Hersch, “Multi-Color and ArtisticDithering”, Siggraph'99, Computer Graphics Proceedings, AnnualConference Series, 1999, pp. 425-432. The method allows to synthesizecolor images incorporating as screen dots a fine microstructure capableof representing various shapes such as characters, logos, and symbolsand provides therefore strong anti-counterfeiting features. Thepublication also presents an iterative technique for equilibrating adither array, which is however slow and cumbersome and does not alwaysconverge to yield a satisfying result. A disadvantage of theaforementioned and other known methods is the significant effortrequired to synthesize dither matrices incorporating the desiredmicrostructure shapes. These efforts require the skills of a computerscientist for building 3D functions, discretizing them, renumbering theresulting dither values and applying to them an equilibration process.

An additional method for creating microstructures within an image relieson a large dither matrix whose successive threshold levels represent themicrostructure and uses standard dithering to render the final image(see Oleg Veryovka and John Buchanan, Halftoning with Image-Based DitherScreens, Graphics Interface Proceedings, years 1988-1999, Ed. ScottMacKenzie and James Stewart, Morgan Kaufmann Publ. orhttp://www.graphicsinterface.org/proceedings/1999/106/). In this paper,the authors show how to build a dither matrix from an arbitrarygrayscale texture or grayscale image. They mainly apply histogramequilibration to ensure a uniform distribution of dither thresholdlevels. Texture control is obtained by error-diffusion. However, whiletheir method allows to incorporate text within the microstructure, thetypographic character shapes do not vary according to intensity, i.e.the character shapes do not become thin or fat depending on the localintensity. Their method is restricted to black-white or single colortarget images. The authors do not provide a method to construct a dithermatrix starting from a bi-level bitmap incorporating the microstructureshapes.

A further method of embedding a microstructure within an image isdescribed in provisional U.S. patent application No. 60/312,170 (filedAug. 14, 2001, inventor Huver Hu, available at Web sitehttp://www.amgraf.com/), which teaches how to transform a grayscale seedimage or a bi-level seed image into an array of dot ranking values(similar to a dither matrix) to be used by a PostScript Interpreter forsynthesizing the final image incorporating the microstructure. Thismethod is however limited to black-white or to single color outputimages (bi-level images). In addition, the seed image is preferably agrayscale image (FIG. 10 of patent application No. 60/312,170). Withbi-level seed images, the generated microstructure is limited to rathersimple shapes (FIG. 13 of patent application No. 60/312,170), sinceshapes grow at increasing darkness levels from a user specified growthcenter to the shape given by the bi-level seed image. The shape does notgrow beyond 60% darkness: darker levels are produced by the growth of aseparate superimposed geometric mask (e.g. a triangle, visible on alldark parts of the wedges in FIGS. 2, 12 and 13 of patent application No.60/312,170). Furthermore, a manual interactive intervention is requiredto transform a seed image into an array of dot ranking values.

Another approach for embedding information within a color image relieson the modification of brightness levels at locations specified by amask representing the information to embed, while preserving thechromaticity of the image (see U.S. Pat. No. 5,530,759). However, sincethe embedded information is not really used to construct the globalimage, it cannot be considered a microstructure. If the embeddedinformation incorporates large uniform surfaces, the global image may besubject to significant changes and the embedded information may becomevisible from a large distance. In addition, the mask is fixed, i.e. itsshape does not vary as a function of the local intensity or color.

One further related invention disclosed in U.S. Pat. No. 5,995,638teaches a method for authenticating documents comprising a basic screenmade of microstructures and a revealing screen for creating moireintensity profiles of verifiable shapes. U.S. patent application Ser.No. 09/902,445 describes a similar method, where however the basicscreen and the revealing screen may undergo geometric transformations,yielding screens of varying frequencies.

The incorporation of microstructures in images has applications not onlyin the field of generation of artistic images, but also in the field ofgeneration of documents that require protection against counterfeiting.It is known to incorporate microstructures as a security feature incertain printed commercial instruments, such as bank notes, usingprofessional printers and printing techniques on special substrates.

A primary consideration in the generation of printed commercialinstruments, such as bank notes, vouchers, transportation tickets,entertainment event tickets and other tickets, coupons or receiptsbearing or representing a commercial value, is to provide sufficientsafeguards against forgery. The required degree of difficulty inproducing a forgery will depend above all on the value, the duration ofvalidity and the generality of the commercial instrument. For example,bank notes which are not related to any specific event and remain validfor many years, require security features that are extremely difficultto reproduce. On the other hand, tickets of relatively limited duration,for example transportation tickets, such as train tickets valid on acertain day for a certain destination, or theatre tickets for a specificshow, require lower level security features, as long as they ensure thatthe instrument is difficult to reproduce in the remaining time to theevent or requires excessive technical means or human effort incomparison to the value of the commercial instrument.

Verification of the authenticity of many commercial instruments is oftenbased on a visual control. Although it is easy to provide commercialinstruments with unique security features, such as encrypted bar codesor other codes, their verification entails the use of electronicprocessing means that are unpractical or inefficient in many situations.

In commercial instruments relying primarily on a visual control ofauthenticity, a common security feature is the provision of specialsubstrates that are difficult or too costly to reproduce for a potentialforger in relation to the underlying value of the commercial instrument.A disadvantage of the use of special substrates or special printingtechniques is that they do not allow the generation of commercialinstruments at sites that are not under the issuer's control, whetherdirectly or indirectly.

In view of the wide-spread use of communications networks, such as theinternet or local area networks, there is a demand for enabling thegeneration of visually verifiable printed commercial instruments, suchas transportation tickets and entertainment event tickets, at thebuyer's site, for example at home with a PC and standard printer.

In international patent application WO 00/67192, a method of generatinga commercial instrument with certain visually verifiable securityfeatures for printing on a standard printer is described. In theaforementioned application, data relevant to the commercial instrumentare manipulated in accordance with predetermined rules to generate apattern which is visually recognizable to an informed person. Thesecurity against forgery of an instrument generated according to thelatter method relies on the potential forger's ignorance of thepredetermined rules.

Reliance on predetermined rules has a number of disadvantages. Firstly,the rules must be communicated to persons responsible for controllingauthenticity, which becomes impractical where many controllers areinvolved. Secondly, the rules must result in features that are visuallyrecognizable, with the consequence that a potential forger could, on thebasis of a number of commercial instruments, be able to deduce the ruleswith a sufficient degree of approximation to generate forgeries usingdifferent data. In this regard, it should be noted that the relativelysophisticated image creation and editing software widely available andfor use on PC provide the forger with fairly powerful tools to reproduceimages and text manipulated in order to emulate visually recognizablepatterns provided on authentic commercial instruments on the basis ofpredetermined rules as described in international application WO00/67192.

SUMMARY OF THE INVENTION

An object of this invention is to provide images incorporating amicrostructure that may be generated efficiently.

Another object of this invention for certain applications is to provideimages that are difficult to counterfeit, in particular for use indocuments as a security feature.

It is advantageous in certain applications to provide imagesincorporating a microstructure that may be rapidly generated.

It is advantageous in certain applications to provide imagesincorporating a microstructure that have a high resolution or a highvisual quality.

It is advantageous in certain applications to provide imagesincorporating a microstructure, that can be animated.

It is also an object of this invention to provide a method of generatingsuch images, and a method of generating documents comprising suchimages. It is also an object of this invention to provide a computersystem to generate such images.

Another object of this invention is to provide a security document, suchas a commercial instrument or certificate, and a method of generationthereof, that is difficult to forge yet enables visual verification ofthe authenticity thereof, and that can be printed with non-professionalprinting systems, such as standard PC printers, or displayed on anelectronic display.

It is advantageous to provide a security document with security featuresthat are easy to verify visually by a verifying person, without the needfor providing such person with restricted information on hidden or codedsecurity features or other information unavailable to uninformedpersons.

It is advantageous to provide a method for generating security documentsthat is able to generate personal and/or event specific instrumentsrapidly, for example comprising information relating to a specificperson, destination or event.

It is further advantageous to provide a method that enables theprinting, or downloading for display on a portable device screen, ofsecure commercial instruments by a customer with access to dataprocessing and database means through a communications network such asthe internet.

Objects of this invention have been achieved by providing a method ofgenerating an image incorporating a microstructure according to claim 1.

Disclosed herein is a method of generating an image incorporating amicrostructure, including

-   -   obtaining an original image;    -   generating a microstructure; and    -   rendering a region or the whole said original image with said        microstructure;        wherein the operation of generating the microstructure includes        an automatic synthesis of microstructure elements from original        microstructure shapes. The microstructure shapes are in an        embodiment described originally in the form of a bi-level        bitmap. The automatic synthesis from bitmaps enables very        efficient creation of images on the fly, which may incorporate        different microstructure shapes, for example based on        information specific to the content of a document in which the        image is used. In addition, thanks to a parametrized        transformation carried out at microstructure image rendering        time, different instances of the same microstructure image can        be rendered on the fly. An important advantage of the presented        automatic dither array synthesis method is its ability to ensure        that the microstructure incorporated into an image or a security        document remains visible at nearly all intensity levels (from        10% to 90% darkness in most cases). A high quality and secure        image incorporating a microstructure can thus be generated.

Objects of this invention have been achieved by providing a method ofgenerating an image incorporating a microstructure according to claim 3.

Also disclosed herein is a method of generating an image incorporating amicrostructure, including

-   -   obtaining an original image;    -   generating a microstructure; and    -   rendering said original image with said microstructure;        wherein the microstructure includes a low frequency        microstructure generated from low frequency microstructure        elements, and a high frequency microstructure generated from        high frequency microstructure elements, whereby the low        frequency microstructure elements are larger than the high        frequency microstructure elements. The two levels of        microstructure advantageously provides an image that is very        difficult to forge. It is also possible to have further levels        of microstructure incorporated in the image.

The microstructure may be composed of text, graphic elements andsymbols. The microstructure whose shapes vary according to intensity andcolor protects the security document's elements such as text,photographs, graphics, images, and possibly a background motif. Sincethe security document is built on top of the microstructure, documentelements and microstructure elements cannot be erased or modifiedwithout introducing discontinuities in the security document.Furthermore, thanks to transformations having the effect of warping themicrostructure into different orientations and sizes across the securitydocument, individual microstructure elements cannot be simply copied andinserted elsewhere.

The present disclosure also teaches how to equilibrate an imageincorporating a microstructure (hereinafter also called: “microstructureimage”) or a security document with the help of a high-frequency ditherarray. This high-frequency dither array may incorporate a second levelmicrostructure providing an additional level of protection.

Further disclosed herein are microstructure images and securitydocuments with a microstructure rendered in black/white, color, orpossibly rendered partly with non-standard inks, or special inks such asfluorescent inks, phosphorescent inks, metallic inks, iridescent inks orultra-violet inks. A mask whose shape expresses a visual message (e.g. abold text string or a symbol) may specify the part of the targetdocument to be rendered with a special ink. Under given observationconditions (e.g. type of light, viewing angle), the special ink ishidden. Under other observation conditions, the special ink has theeffect of making the mask shape (e.g. the text or symbol) clearlyvisible. For example at a certain viewing angle, the part covered by thespecial ink is hidden and when seen from another angle, it becomesapparent.

Further disclosed herein is an animated microstructure image formed by amicrostructure evolving over time, where from far away mainly the imageis visible and from nearby mainly the evolving microstructure isvisible. Such an animated microstructure image is displayed as asuccession of image instances, each image instance differing fromprevious image instances by the microstructure-evolution. Thismicrostructure evolution is determined by a parametrized transformation,whose parameters change smoothly as a function of time.

Further disclosed herein is a method allowing to combine an originalimage, respectively a conventionally halftoned original image with amicrostructure image, thereby providing within the target image more orless weight to the microstructure. This allows to create target images,where thanks to a multi-valued mask, the relative weight of themicrostructure may at certain places, slowly reduce and disappear. Inthe case of an animated microstructure image, the mask specifies thepart of the image to be rendered with an animated microstructure and thepart which is being left without microstructure. With a multi-valuedmask, the appearance of the microstructure can be tuned to be strong oron the contrary at the limit of what can be perceived by a human eye ata normal observation distance. In addition, mask values evolving overtime yield apparent changes in the embedded microstructure appearanceproperties such as the visibility, location or spatial extension of theembedded microstructure within the image.

In a preferred embodiment, original microstructure shapes are embeddedwithin a bilevel bitmap, and the microstructure is embodied by a ditherarray. Starting from the bitmap incorporating the microstructure shapes,the dither array can be automatically generated. A black-white or colortarget image (or security document) is synthesized by dithering anoriginal image with the dither array and by possibly equilibrating theresulting dithered original image.

Also disclosed herein is a computing system for synthesizing securitydocuments comprising a an interface operable for receiving a request forsynthesizing a security document, a software preparation module operablefor preparing data files from document information and a documentproduction module operable for producing the security document. Thepreparation of data files may comprise the generation of an originaldocument image, of microstructure shapes and possibly of transformationparameters. Producing the security document system comprises thesynthesis of a microstructure and the synthesis of the security documentwith that microstructure.

Further disclosed herein is a computing system for synthesizing imagescomprising an interface operable for receiving a request forsynthesizing a microstructure image and comprising a software productionmodule operable for producing the microstructure image. The requestcomprises an original image and microstructure shapes. Themicrostructure image is produced by the production module by firstsynthesizing a microstructure and then by synthesizing themicrostructure image incorporating that microstructure.

Further disclosed herein is a computing system capable of displaying atarget image with an embedded microstructure evolving over time, wherefrom far away mainly the image is visible and from nearby mainly theevolving microstructure is visible. The computing system comprises aserver computing system and a client computing and display system. Theclient computing and display system receives from the server computingsystem as input data an original color image, microstructure data andmicrostructure evolution parameters. The client computing and displaysystem synthesizes and displays the target image with the embeddedmicrostructure on the fly.

Other objects of this invention have been achieved by providing a methodof generating a security document according to claims 34 or 35.

Disclosed herein is a method of generating a security document forprinting or display, including the steps of:

-   -   selecting, retrieving or composing an original image;    -   selecting or retrieving information specific to a person, an        event or transaction to which said security document relates;    -   generating a microstructure comprising readable microstructure        elements providing information on said person, event or        transaction;    -   rendering said original image with said microstructure image.

The microstructure may advantageously be generated as a dither matrix,automatically synthesized from microstructure shapes such as bitmapelements.

The microstructure may be rendered with the image by rendering methodsdescribed above or by a halftoning process, whereby the pixels of thedither matrix are compared with the pixels of the background image and,for example, if the pixel of the background image has a grey levelgreater than the inverse grey level of the dither matrix, then the pixelis printed as white, otherwise it is printed as black. The rendering ofthe microstructure and image may further comprise a step of balancingthe halftoned image.

Advantageously, in view of the rendering process, the event ortransaction specific information is extremely difficult to separate outof the background or original image and is therefore difficult toreplace with other information in view of producing forgeries. Themicrostructure dither matrix may advantageously comprise letters and/ornumbers, such that the event or transaction specific information may beprovided in the form of words or numbers. This enables information, suchas the date, the price, the destination, the seat number, personalidentification, credit card number, ticket transaction number or anyother information specific to the event or transaction to form part ofthe microstructure image. The microstructure dither matrix may alsocomprise other characters, graphical elements, logos and other specialdesigns.

The original image may advantageously comprise a photographicrepresentation or portrait of the customer, in addition to a backgroundimage that may be changed from time to time, the images being merged orsuperposed. The original image may further comprise written tickettransaction information.

Further objects and advantageous aspects of this invention will beapparent from the following detailed description of embodiments of thisinvention with reference to the accompanying figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a dither matrix, where the microstructure is given by thesequence of dither threshold levels, represented in the figure as graylevels;

FIG. 1B shows an enlargement of a part of the dither matrix of FIG. 1Ademonstrating how the dither threshold levels define the microstructure;

FIG. 2 shows uniform intensity patches dithered with the dither matrixof FIG. 1;

FIG. 3 shows an image overlaid with a warping grid;

FIG. 4 shows a mask specifying the parts of the image to be renderedwith microstructures (in black);

FIG. 5 shows one instance of a microstructure image obtained bymulticolor dithering of the original image shown in FIG. 3;

FIG. 6 shows other instances of a microstructure image;

FIG. 7A shows schematically a comparison between an input intensitysignal (or image) P(x) and a dither threshold value G(x) and accordingto that comparison, the setting of a foreground or background color;

FIG. 7B shows relative intensities d_(a), d_(b), d_(c), and d_(d) ofcolors C_(a), C_(b), C_(c), and C_(d);

FIG. 7C shows the conversion of relative intensities d_(a), d_(b),d_(c), and d_(d) of colors C_(a), C_(b), C_(c), and C_(d) intocorresponding surface coverages;

FIG. 8 shows a diagram of elements useful for creating images withtransformed microstructures;

FIG. 9A shows schematically an original image;

FIG. 9B shows schematically a dither matrix paving an original dithermatrix space;

FIG. 10A shows a warping grid laid out in a transformed dither matrixspace;

FIG. 10B shows the grid of FIG. 10A, warped and laid out on top of thetarget image;

FIG. 11A shows a mask specifying the part of the target image to berendered;

FIG. 11B shows one instance of the target image rendered with atransformed microstructure;

FIG. 12 shows the warping transform T_(w)(x,y) mapping from target imagespace to the transformed dither matrix space and the transformationT_(t)(u,v) mapping from the transformed dither matrix space into theoriginal dither matrix space;

FIG. 13A shows a rectangular grid and the warped rectangular gridspecifying the warping transform between target image space andtransformed microstructure space;

FIG. 13B shows a microstructure in the transformed microstructure space;

FIG. 13C shows the same microstructure in the target image space, warpedby the warping transformation defined according to FIG. 13A;

FIG. 14A shows a one-dimensional color CMY image with cyan, magenta andyellow color intensities varying as function of their position on thex-axis;

FIG. 14B shows schematically comparisons between the CMY inputintensities of the image of FIG. 14A and a dither threshold value G(x)and according to these comparisons, the setting of the resulting basiccolors (cyan, magenta and yellow);

FIG. 14C shows the colors resulting from the superposition of the basiccolors set according to the comparison of FIG. 14A;

FIG. 15A shows a one-dimensional color CMY image with cyan, magenta andyellow color intensities varying as function of their position on thex-axis;

FIG. 15B shows schematically the comparison between the cyan inputintensity of the image of FIG. 15A and a dither threshold value G(x) andaccording to this comparison, the setting of the resulting basic cyancolor;

FIG. 16A shows a dispersed-dot two-dimensional dither matrix;

FIG. 16B shows the one-dimensional dithering of constant mask valuesp(x) with 1D dither matrix values D(x) and the resulting spatialdistribution of microstructure image color values C and original imageresampled color values C_(r);

FIG. 17 show the application of a thinning operator to a bitmap withtypographic character A and the resulting ordered list L1 of coordinatesets S1, S2, S3 representing successively erased discrete contours andthe remaining skeleton;

FIGS. 18A and 18B show the thinning steps allowing to obtain theskeleton of character A;

FIGS. 19A and 19B show the dual bitmap of discrete character A;

FIGS. 20A and 20B show the thinning steps allowing to obtain theskeleton of the dual bitmap;

FIGS. 21 and 22 illustrate the two first steps of the alternateddilation algorithm;

FIG. 23 shows the thinning steps applied to the dual bitmap (dual bitmapthinning);

FIG. 24 shows an example of an image rendered without equilibration;

FIG. 25 illustrates the application of a low-pass filter on the ditheredimage and the comparison with the original picture yielding a deltamap;

FIG. 26 is a flow diagram showing the equilibration of a ditheredpicture by post-processing;

FIG. 27 shows an example of a high frequency artistic microstructureused to equilibrate the low frequency microstructure;

FIG. 28 illustrates the low-frequency (LF) dither array, the highfrequency (HF) dither array and the mixed dither array;

FIG. 29A shows the resulting mixed dither array and its application todither a gray wedge;

FIG. 29B shows an enlargement of a constant intensity patch ditheredwith the resulting mixed dither matrix, at a 50% midtone;

FIG. 30 shows an original image;

FIG. 31 shows the same image dithered only with the low-frequency dithermatrix;

FIG. 32 shows the same image, dithered and equilibrated by postprocessing;

FIG. 33A illustrates dither matrix synthesis by alternated dilation anda corresponding dithered gray wedge;

FIG. 33B illustrates dither matrix synthesis by dual erosion and acorresponding dithered gray wedge;

FIG. 34 shows an example of a wedge where from a darkness of 25%, thebackground grows and starts surrounding the foreground shape (Hebrewletters), leaving even at a high darkness a small white gap betweenshape foreground and shape background;

FIG. 35A shows a mask incorporating a visual message;

FIG. 35B shows a microstructure image at observation conditions wherethe mask shape within the microstructure image is clearly revealed;

FIG. 36 shows a diploma incorporating a microstructure containing thename of the document holder and the name of the issuing institution;

FIG. 37 shows a computing system comprising a preparation softwaremodule operable for the preparation and a production software moduleoperable for the production of a security document;

FIG. 38 shows a computing system comprising a production software moduleoperable for the production of a microstructure image;

FIG. 39 shows a server computing system transferring to a clientcomputing and display system an input color image, a dither matrix, ananimation transformation, a warping transformation, a set of basiccolors and a mask layer;

FIG. 40 shows a server system interacting with a designer program or adesigner applet running on a client computer;

FIG. 41 shows a Web page incorporating an animated microstructure image;

FIG. 42 is a schematic illustration of a distributed data processingsystem for implementing a method of generating a printed commercialinstrument according to this invention;

FIG. 43 is a flow-chart describing in a simplified manner various stepsof a method according to an embodiment of this invention;

FIG. 44 is a schematic illustration similar to FIG. 42 of an enterpriseserver system for implementing a method of generating a printingcommercial instrument according to this invention;

FIG. 45 is a schematic illustration similar to FIGS. 42 and 42 of alocal or stand-alone server system for implementing a method ofgenerating a printing commercial instrument according to this invention;

FIG. 46 is a flowchart illustrating a procedure for creating acommercial instrument image according to this invention;

FIG. 47 is an illustration of the transformation of a microstructureimage to a microstructure dither matrix (graphically represented);

FIG. 48 is an illustration of a process of rendering a contextual imageand a dither matrix by a halftoning process;

FIG. 49 is an illustration of the balancing of a halftonedmicrostructure image with the contextual image to form an image forprinting according to this invention;

FIGS. 50 a to 50 g are various graphical representations ofalphanumerical character of a microstructure dither matrix according tothis invention;

FIG. 51 is a flow-chart illustrating a procedure for visual verificationof a printed commercial instrument according to this invention;

FIG. 52 is an illustration of an example of a printed commercialinstrument generated with a method according to this invention;

FIG. 52 a is a detailed view of part of the image of FIG. 52;

FIG. 52 b is a detailed view of part of the image of FIG. 52 a; and

FIG. 53 is an illustration representing an example of the application ofa microstructure dither matrix on an image.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses security documents and methods forgenerating them, where the document information (text, photograph,graphics, images, background, hereinafter called “document elements”) isformed by microstructures having shapes varying with the intensity ofthe document elements. In addition, the microstructure itself maycomprise valuable information, such as the name of the document holder,the type of the document, its validity or any other information relevantto check the authenticity of the document (for example a code expressingopen or hidden document information). The same microstructure maycontinuously cover several document elements of the same securitydocument. Its continuity makes therefore the replacement of individualdocument elements by faked elements very difficult to achieve.

The methods described in the present invention can also be used togenerate artistic images, graphic designs or posters incorporating atleast two layers of information, one at the global level and one at thelocal level.

Furthermore, since these methods can generate multiple instances of thesame global image by simply varying the microstructure according to aparameter dependent transformation, images with differentmicrostructures or images with a microstructure evolving over time canbe synthesized, as disclosed in the parent U.S. patent application Ser.No. 09/902,227 (filed Jul. 11, 2001, by R. D. Hersch and B. wittwer, dueassignee: EPFL).

In the following description of the invention, documents or documentelements to be rendered with microstructures are called “documentimages” or simply “images”. We use the words “document”, “documentimage” and “image” interchangeably. A document, a document image orsimply an image are represented, at least partly, as an array of pixels,each pixel having one intensity value (gray) or several intensity values(color, e.g. CMY intensities). A target document incorporating amicrostructure, is called “security document”, “target image”,“microstructure image” or when the context allows it, simply “image”.Within a security document, or within a target image, at least part ofthe security document, respectively of the target image, is formed by amicrostructure.

The term “local intensity” is generic and means either one localintensity or several local intensities as is the case with images havingmultiple channels such as color images. Often we use the term “darkness”instead of intensity when examples printed in black and white are shown.In these cases, the darkness indicates the relative percentage of theprinted part, i.e. the black ink. It is equivalent to the term “basiccolor intensity” which also gives the relative percentage of acorresponding basic color appearing on the support (e.g. a printed basiccolor).

The term “image” however characterizes not only documents, but alsoimages used for various purposes, such as illustrations, graphics andornamental patterns reproduced on various media such as paper, displays,or optical media such as holograms, kinegrams, etc . . . . Both inputand target images may have a single intensity channel (e.g. black-whiteor single color) or multiple intensity channels (e.g color images). Inaddition, target images may incorporate non-standard colors (i.e colorsdifferent from cyan, magenta, yellow and black), for example fluorescentinks, ultra-violet inks as well as any other special inks such asmetallic or iridescent inks.

In principle, the Artistic Screening method described in the section“Background of the invention” can be applied for generating imagesincorporating information at the microstructure level. It generatesmicrostructures whose shapes vary according to the local intensity.However, since Artistic Screening is restricted to bi-level images andrequires a significant design effort in order to create contours ofartistic screen elements at different intensities, the preferred methodfor synthesizing images with embedded microstructures is based either onstandard dithering or on the Multicolor Dithering method cited above.

Hereinafter, the term dithering without the adjective “standard” or“multicolor” refers to both standard dithering and Multicolor Dithering.Standard as well as Multicolor Dithering make use of a dither matrix,whose distribution of dither threshold values represents themicrostructure that will be part of the resulting target image (FIG. 1Aand FIG. 1B). Both standard dithering and Multicolor Dithering reproducean input image (also called original or global image) in such a way thatwhen seen from nearby, mainly the microstructure embedded into theglobal image is visible, whereas when seen from far away, mainly theglobal image is visible (FIG. 5).

Hereinafter the terms “dither matrix” and “dither array” are usedinterchangeably. A dither array is composed of “cells” incorporating“dither threshold values” or simply “dither values”. As known in theart, small and middle size dither matrices tile the target image plane.However, the dither matrices used in the present invention may be verylarge, possibly as large or larger than the target image.

The term “automatic dithering” refers to the full process of (i)creating automatically a dither matrix from an image or bitmapincorporating the microstructure shapes, (ii) rendering a targetdithered image by either standard dithering or multicolor dithering, and(iii) possibly applying a postprocessing step for target imageequilibration

Some techniques used in the present invention, such as a parameterdependent transformation T_(t) specifying instances of themicrostructure and a warping transformation T_(w) are also used in theparent co-pending patent application U.S. Pat. No. 09/902,227, filedJul. 11, 2001, by R. D. Hersch and B. Wittwer. However, this parentco-pending application is centered on the generation of animatedmicrostructure images, i.e. image sequences and animations, whereas thepresent invention deals mainly with still images and security documentsincorporating a microstructure. However, the method for automaticsynthesis of dither matrices disclosed in the present invention alsogreatly facilitate the creating of images with an animatedmicrostructure.

Standard Dithering

Standard dithering converts an intensity into a surface percentage. Anintensity P(x) of foreground color C is compared with a dither thresholdvalue G(x) and according to the comparison (see FIG. 7A), if P(x)>G(x),the corresponding location x is set to the foreground color and ifP(x)<=G(x), it is left as background color. FIG. 1A gives an example ofa large dither matrix incorporating the microstructure “GET READY”; FIG.1B shows an enlarged part of it and FIG. 2 represents the reproductionof uniform single color images at 20%, 40%, 60% and 80% foreground colorintensity (the foreground color is represented as black). For moreexplanations on standard dithering, see H. R. Kang, Digital ColorHalftoning, SPIE Press and IEEE Press, chapter 13, 213-231.

Multicolor Dithering

Multicolor Dithering is an extension of standard dithering. InMulticolor Dithering, a color C is rendered by a barycentric combinationof several basic colors, for example the combination of 4 colors C_(a),C_(b), C_(c), and C_(d). Their respective relative weights are d_(a),d_(b), d_(c) and d_(d) (FIG. 7B). Multicolor Dithering converts theserelative weights into relative surface coverages. Multi-color ditheringconsists of determining the position of threshold value G in respect tointervals 0 . . . d_(a), d_(a) . . . (d_(a)+d_(b)), (d_(a)+d_(b)) . . .(d_(a)+d_(b)+d_(c)), (d_(a)+d_(b)+d_(c)) . . . 1, (see FIG. 7C).According to the interval within which G is located, the dithered targetimage color C(x,y) will take value C_(a), C_(b), C_(c), or C_(d) (seeFIG. 7C, color values along the x-axis). More precisely, if 0<=G<d_(a),C(x,y)=C_(a); if d_(a)<=G<(d_(a)+d_(b)), C(x,y)=C_(b); if(d_(a)+d_(b))<=G<(d_(a)+d_(b)+d_(c)), C(x,y)=C_(c); and if(d_(a)+d_(b)+d_(c))<=G<=1, C(x,y)=C_(d). Best results are obtained byordering the 4 basic colors C_(a), C_(b), C_(c), and C_(d) located atthe vertices of a tetrahedron according to their increasing CIE-LABlightness values L*.

The method for generating images formed by microstructures requires thedefinition of the following elements (see FIG. 8):

-   -   an original image (also called global image);    -   an original microstructure, preferably embodied as a dither        matrix;    -   color information necessary for rendering the target        microstructure image (optional);    -   an instance dependent transformation T_(t) specifying instances        of the microstructure evolving as a function of a parameter t;    -   a warping transformation T_(w) specifying a warping between the        instantiated or initial microstructure and the warped        microstructure (optional);        and optionally a mask specifying the global image portions which        are to be rendered with microstructures as well as a possible        blending between original image and pure microstructure Image,        the blending allowing to specify microstructure appearance        properties such as visibility, position and spatial extension of        the microstructure.

The original image is located in an original image space (x′,y′), theoriginal microstructure is located in an original microstructure space(also called original dither matrix space) (x″,y″), the transformedmicrostructure is located in a transformed microstructure space (alsocalled transformed dither matrix space) (u′,v′), and the targetmicrostructure image is located in the target microstructure imagespace, also simply called target image space (x,y).

Hereinafter, original image (x′,y′) may stand for original image space(x′,y′), original microstructure (x″,y″) may stand for originalmicrostructure space (x″,y″), transformed microstructure may stand fortransformed microstructure space (u′,v′) and target image (x,y) maystand for target image space (x,y).

The microstructure may represent a text, a logo, a symbol, an ornamentor any other kind of visual motif. Furthermore, the microstructure maycombine several items, e.g. several symbols either identical ordifferent, or a freely chosen combination of text, logos, symbols andornaments. In the preferred cases of standard dithering and MulticolorDithering, the microstructure is defined by a dither matrix whosesuccession of dither threshold levels represent the desired visualmotifs (FIG. 1B).

The parameter dependent geometrical transformation T_(t) may either be aparameter-dependent geometric transformation (e.g. translation,rotation, scaling, linear transformation, non-linear geometrictransformation) or any other parametrized transformation creating fromat least one microstructure a transformed microstructure whose shapevaries as a function of one or several parameters. By modifying theparameters of the transformation T_(t), one may create differentinstances of the same image and with the same microstructureinformation. This allows to creating variations of a security documentaccording to relevant document information, such as its issued date, itsvalidity or its document category. In a preferred embodiment, thetransformation T_(t) provides the mapping between the transformed dithermatrix space (u,v) and the original dither matrix space (see FIG. 12).

The warping transformation T_(w)(x,y) which provides a warping betweenthe target image space (x,y) and the transformed dither matrix space(u,v) may either be given by a formula allowing to obtain from alocation (x,y) in the target image space the corresponding location(u,v) in the transformed dither matrix space or by a program functionreturning for a given (x,y) coordinate in the final target image spacethe corresponding location (u,v) in the transformed dither matrix space(see FIG. 12, transformation T_(w)(x,y)). Alternately, the warpingtransformation may be specified piece wise, by allowing the designer tospecify a rectangular grid of control points and by allowing him to warpthis grid as shown in FIG. 13A.

The color information necessary for rendering the target transformedmicrostructure image may comprise either an indication of which originalimage color layers {C_(i)} are to used for rendering the targettransformed microstructure image or the specification of a set of basiccolors {C_(i)} comprising possibly colors different from red, green andblue, cyan, magenta, yellow, white and black, with which the targetimage is to be synthesized. Colors which are members of the set ofcolors {C_(i)} used for microstructure image rendering are calledhereinafter “basic colors”. A basic color is a color reproducible on theselected support (paper, plastic, metal, partly or fully transparentsupport, optical device). For example on paper, basic colors may bestandard cyan, magenta, yellow and black, non-standard colors, (e.g. aPantone color such as color Pantone 265C) and special inks such asmetallic inks and iridescent inks (optically variable inks).Furthermore, basic colors also comprise opaque inks, which may offer acertain protection against counterfeiting attempts when printed forexample on transparent support.

In the case of a mask with more than two levels of intensity, the mask'svalues specify a blending between the image rendered withmicrostructures, for example a dithered image (standard or multi-color)and the color obtained by simple resampling of the original imageaccording to the target's image size and resolution. Such a blendingallows to produce less pronounced microstructures.

The method for generating a microstructure target image is formulatedbelow in general terms so as to encompass all methods capable ofgenerating information at the microstructure level. However, in apreferred embodiment, either standard dithering or multicolor ditheringis used.

The method for generating a target image with an embedded microstructurecomprises the following steps (see FIG. 8):

-   -   (a) definition of elements required for generating the target        image, i.e. an original image, an original microstructure (in a        preferred embodiment, an original dither matrix), possibly color        information specifying a set of basic colors {C_(i)} used for        rendering the target microstructure image, a parameter-dependent        transformation, possibly a warping transformation and a mask;    -   (b) traversing the target image (x,y) pixel by pixel and row by        row, determining corresponding positions in the original image        (x′,y′), in the transformed microstructure (preferred        embodiment: transformed dither matrix) (u,v), in the original        microstructure (preferred embodiment: original dither matrix)        (x″,y″) and in the mask;    -   (c) obtaining from the original image position (x′,y′) the color        C_(r) to be reproduced, from the original microstructure        (preferred embodiment: original dither matrix) space position        (x″,y″) the rendering information (preferred embodiment: the        dither threshold value G) and from the current mask position the        corresponding mask value p;    -   (d) carrying out the target image rendering algorithm (preferred        embodiment: standard dithering or multicolor dithering) and        determining output color C, possibly from the set of basic        colors {C_(i)};    -   (e) according to the mask value p, performing a blending between        rendered (preferred embodiment: dithered) output color C and        original image color C_(r). In the case of simple printers        capable of printing only a limited number of distinct color        intensities, color C_(r) is rendered by its equivalent halftone        colors C_(pqrs) obtained by a conventional halftoning technique        (e.g. using blue noise masks, as described in K. E.        Spaulding, R. L. Miller, J. Schildkraut, Method for generating        blue-noise dither matrices for digital halftoning, Journal of        Electronic Imaging, Vol. 6, No. 2, April 1997, pp 208-230,        section 4 “Blue Noise Matrices for Color Images”).

If the mask value p indicates that the present image location does notneed to be rendered with transformed microstructures, then step (c) ismodified to directly put color C_(r), respectively its equivalenthalftone colors C_(pqrs), to be reproduced in the target image and steps(d) and (e) are skipped. If the mask is inexistent, then the whole imageis reproduced with transformed microstructures.

The original image may be a simple RGB color image stored in any knownformat. The microstructure (in a preferred embodiment: the dithermatrix) is either precomputed and ready to use or has been created asdescribed in the sections below, starting from section “Automaticsynthesis of a dither matrix”.

Generation of Microstructure Images by Standard Dithering

It is however possible to generate images with microstructures byapplying the standard dithering method with a large dither matrixincorporating the microstructure shapes independently to one or severalbasic colors. A basic color may be selected from the set of cyan,magenta and yellow or any other set of colors by which the image isdescribed. One may apply standard dithering to one, several or all basiccolors. As an example, one may apply standard dithering separately tothe cyan, magenta and yellow layers of an image (FIG. 14A and FIG. 14B)and display the resulting target image by superposing the dithered cyan,magenta and yellow layers. The resulting target image will thus berendered with cyan, magenta, yellow, red (overlap of yellow andmagenta), green (overlap of cyan and yellow), blue (overlap of cyan andmagenta) and black (overlap of cyan, magenta and yellow), see FIG. 14C.

Instead of applying standard dithering to cyan, magenta and yellow as inthe previous example, one may also apply standard dithering to one ofthe color layers, for example the predominant color layer or the colorlayer dominant in the image part where one would like to insert themicrostructure. For example, in order to insert a microstructure in thesky, one may choose to apply standard dithering to the cyan layer (FIG.15B) and reproduce the other color layers by conventional methods suchas cluster-dot screening or error-diffusion. In that case, target imagepixels are composed of a cyan color layer obtained by standard ditheringwith a large dither matrix incorporating the microstructure shapes andmagenta and yellow layers are reproduced with a conventional halftoningmethod.

Generation of Microstructure Images by Multicolor Dithering

In the preferred embodiment of generating microstructure images byMulticolor Dithering, the method comprises initialization steps,rendering steps and an image printing step.

The initialization steps comprise (a) the initialization for the colorseparation of the original image (e.g. RGB) according to the selectedset of basic colors, (b) the creation of a data structure facilitatingthe color separation, (c) carrying out the color separation andassociating in a color separation map to each target color image pixelthe basic colors with which it is to be color dithered and theirassociated basic colors weights, (d) associating in a warping transformmap to each location (x,y) within the target image space a pointer tothe corresponding location in the transformed dither matrix spaceaccording to the user defined warping transformation. Steps (b), (c) and(d) are useful for speeding up image rendition, especially when applyingthe same warping transformation on successively generated target images.As a variant, one may choose to carry out the color separation andpossibly the warping transform during image rendering.

Several methods for carrying out the color separation exist: one maysolve the Neugebauer equations for the set of output colors (see forexample H. R. Kang, Color Technology for Electronic Imaging Devices,SPIE Optical Engineering Press, 1997, Chapter 2, Section 1, pp. 34-40)or place the output colors in an output color space, e.g. CIE-XYZ andtetrahedrize that space (see S. M. Chosson, R. D. Hersch, Visually-basedcolor space tetrahedrizations for printing with custom inks, Proc. SPIE,2001, Vol. 4300, 81-92). In that case, the preferred data structurefacilitating the color separation is a 3D grid data structure pointingto the tetrahedra intersecting individual grid elements.

In the case that the selected basic colors are located in a rectilineargrid, the tetrahedrization is straightforward : each cube or rectilinearvolume element comprising 8 vertices can be decomposed into 6 tetrahedra(see H. R. Kang, Color Technology for Electronic Imaging Devices, SPIEOptical Engineering Press, 1997, Section 4.4 Tetrahedral interpolation,pp 70-72). If the designer is allowed to choose any set of basic colorsor when non-standard or special inks are used, the tetrahedrization isslightly more complex, but can be carried out without difficulty withprior art methods (see for example the book Scientific Visualization :Overviews, Methodologies, and Techniques, by Gregory M. Nielson, HansHagen, Heinrich Muller, Mueller (eds), IEEE Press, Chapter 20, Tools forTriangulations and Tetrahedrizations and Constructing Functions Definedover Them, pp. 429-509).

In the case that the color separation is carried out bytetrahedrization, each target image pixel color is rendered by 4 basiccolors, members of the selected set of the basic colors. For computingthe 4 basic colors associated with each target image pixel (x,y), thecolor C_(r) at the corresponding original image location (x′,y′) isdetermined by resampling, i.e. by interpolating between colors ofneighbouring original image pixels (e.g. prior art nearest neighbour orbi-linear interpolation). Resampled color C_(r) is used to find thetetrahedron which encloses it. The 4 basic colors C_(a), C_(b), C_(c),C_(d) located at the tetrahedron's vertices and their barycentricweights d_(a), d_(b), d_(c), d_(d) allowing to render resampled originalimage color Cr according to C_(r)=d_(a) C_(a)+d_(b) C_(b)+d_(c)C_(c)+d_(d)C_(d) may be stored, possibly together with original imageresampled color C_(r), in a target image color separation map. The basiccolors member of the set {C_(a), C_(b), C_(c), C_(d)} with the largestrelative amounts are called the dominant colors. Security documentelements such as text, graphics or images may be conceived within alimited color gamut so as to ensure that only one or two colors arepredominant across the largest part of that element's surface. This willyield a microstructure, where the dominant colors are thick in darkregions and thin in highlight regions of the security document

The image rendering steps are as follows. For rendering successivetarget image instances of the target microstructure image, for eachtarget image instance, we traverse the target image space pixel by pixelby traversing one pixel row after the other. For each target pixel(x,y), if the target image mask value M(x,y) indicates that multi-colordithering is to be applied, (e.g. M(x,y)<>0), we read from the targetimage color separation map the basic colors and their respectiveweights. We determine the dither threshold value G associated with atarget pixel (x,y) by obtaining the pointer to the correspondinglocation (u,v) in the transformed dither matrix space, for example byaccessing the warping transform map created in the initialization phaseand from there, by applying the current transformation T_(t)(u,v), weobtain the current location (x″,y″) within the original dither matrixspace. The threshold value G(x″,y″), the basic colors C_(a), C_(b),C_(c), C_(d) and their respective weights d_(a), d_(b), d_(c), d_(d) areused for multicolor dithering. Multi-color dithering consists ofdetermining the position of threshold value G with respect to intervals0 . . . d_(a), d_(a) . . . (d_(a)+d_(b)), (d_(a)+d_(b)) . . .(d_(a)+d_(b)+d_(c)), (d_(a)+d_(b)+d_(c)) . . . 1. According to theinterval within which G is located, the dithered target image colorC(x,y) will take value C_(a), C_(b), C_(c), or C_(d) (see FIG. 7C andsection “Multicolor dithering” above). In the case that standarddithering is used instead of multi-color dithering, we determine asabove the dither threshold value G and use it to compare it with theintensity of the basic color (or colors) to be dithered and according tothe comparison (see section “Standard dithering” above), use that basiccolor (or colors) to render the current target image pixel (x,y). FIG.15B shows how dithering can be applied to one of the image's color's,namely cyan.

For rendering different target image instances with the same originalimage and the same original microstructure shapes, the parametrizedtransformation T_(t)(x,y) describing the mapping between the transformeddither matrix space and the original dither matrix space may bemodified.

In the case of a mask M(x,y) specifying discrete values representing aproportion p between 0 and 1, the final color C_(f) (x,y) is acombination of the dithered color C(x,y) and of the original color C_(r)(possibly reproduced by a conventional halftoning method), for exampleC_(f) (x,y)=p C(x,y)+(1-p) Cr. Instead of a pixel-wise blending betweenthe dithered image color C(x,y) and the color C_(r) (which would be onlyfeasible on a multi-intensity reproduction device such as a dyesublimation printer), it is possible to apply a spatial blending, i.e.to ensure that only proportion p of neighbouring pixels take thedithered color C(x,y) and proportion (1-p) takes the originalconventionally halftoned color values C_(r). For this purpose, one canuse for example a spatial dispersed dither matrix D(x,y), e.g. Bayer's4×4 dither matrix (FIG. 16A) and use thresholds t=0,1,2 . . . 15 todecide if a pixel should take the original conventionally halftonedcolor value Cr, when p=<t/16 or take the dithered color C when p>t/16.As an illustration of spatial blending, FIG. 16B shows inone-dimensional space the comparison between the proportion p(x) and thedither values D(x): where p(x)>D(x), the corresponding segment (black inFIG. 16B) takes the dithered image color values C(x) and wherep(x)<=D(x), the corresponding segment (white in FIG. 16B) takes theoriginal conventionally halftoned color values C_(r)(x).

The printing step comprises the printing of the generated microstructureimage. It should be noted that the terms “print” and “printing” in thepresent disclosure refer to any process for transferring an image onto asupport, including by means of a lithographic, photographic,electrophotographic, ink-jet, dye-sublimation, engraving, etching,perforing, embossing or any other process.

As an example let us assume FIG. 9A represents the original color image.FIG. 9B represents the dither matrix paving the original dither matrixspace. The parametrized transformation T_(t) maps the transformed dithermatrix within an transformed dither matrix space into the originaldither matrix space. FIG. 10A represents a warping grid laid out overthe transformed dither matrix space. In FIG. 10B, the warped grid isshown in the target image space. The warping transformation T_(w) allowsto map locations from the target image space into correspondinglocations in the transformed dither matrix space. FIG. 11A shows a maskspecifying which part of the original image needs to be rendered bymicrostructures. FIG. 11B shows schematically the rendered target colorimage space, where the part covered by the mask is rendered withmicrostructures. The “LSP” microstructure is obtained thanks to thewarping transformation (FIG. 13A) which transforms for example therepetitive microstructure shown in FIG. 13B into the warpedmicrostructure shown in FIG. 13C.

As real example, FIG. 1. shows a dither matrix comprising the “GETREADY” microstructure shapes. FIG. 2. shows the microstructure obtainedby dithering with constant foreground color intensity levels of 20%,40%, 60% and 80% (the foreground color is shown in black, the backgroundis represented by the paper white). FIG. 3. shows the original image,with a superimposed warping grid (the grid is made of rectangularelements, with one additional diagonal per rectangle defining twotriangles; the triangles are used for the warping transformation). Inthe present case, the warping grid has the effect of shrinking themicrostructure at the bottom and top of the image. FIG. 4 shows thebi-level mask specifying the regions to be rendered with amicrostructure and FIG. 5 shows one instance of the resulting imagecomprising a microstructure in the regions specified by the mask. Onecan easily perceive the microstructure made of the warped “GET READY”shapes. FIG. 6 shows several instances of the rendered microstructureimage, i.e. the rendered microstructure image at different time points.The display of a microstructure image where in successive frames,transformations parameters evolve smoothly over time yields an imagewith a smoothly evolving microstructure hereinafter called “animatedmicrostructure image” or “image with embedded microstructure evolvingover time” or simply “image with animated microstructure”. Thetransformation, also called “animation transformation” moves themicrostructure up and down and at the same time displaces it slowly tothe left. The animation transformation T_(t) of this example has theformx″=s _(x)(u+k _(u) ·i)y″=s _(y)(v+A·cos((s·i+u)360/λ))where i is the number of the current target image instance, s is thewave oscillating speed, k_(u) is the horizontal translation speed, l isthe horizontal period of the microstructure wave, A is its amplitude ands_(x), s_(y) represent respectively horizontal and vertical scalingfactors. The cosinusoidal vertical displacement of the microstructuredepends on its current location u, i.e. there is a phase difference inthe vertical displacement of the microstructure at different horizontallocations. Variables u and v represent respectively the currenthorizontal and vertical coordinates within the transformed dither matrixspace (u, v). An animated microstructure image may be incorporated intoa support formed by an optical device. Such optical devices may compriseholograms, kinegrams or diffractive elements.Use of Color and Microstructures for Strengthening the DocumentProtection

Color images can strengthen the security of documents againstanti-counterfeiting attempts by making it more difficult for potentialcounterfeiters to replace individual document elements or individualmicrostructure elements by other faked elements. One may for examplecreate images with strongly varying colors for the subsequent synthesisof a target color microstructure image by taking as input image agrayscale image, overlaying on top of it a grid and assigning to eachgrid point a chromatic value in a suitable color space, for example avalue for hue (H) and saturation (S) in the HLS color model (see Foley,Van Dam, Feiner, Hughes, Computer Graphics: Principles and Practice,Addison-Wesley, 1999, section 13.3.5: The HLS Color Model, pp 592-595).That grid may be warped as shown in FIG. 13A. The original or possiblythe warped grid define by interpolation (triangular interpolation withintriangles obtained by subdivision of the grid quadrilaterals into pairsof triangles) one hue and saturation value for each pixel of thegrayscale image. The intensity of each pixel of the grayscale image maybe proportionally mapped onto the lightness (L) of the HLS space. Bytransforming back the HLS values of each pixel into RGB and thenpossibly into CMY (C=1-R, M=1-G, Y=1-B) one obtains an original colorimage with strong color variations, which after subsequent ditheringwith a dither matrix incorporating a microstructure will create a targetmicrostructure image with a strongly varying local microstructure color.Such variations, together with the necessity of recreating manuallymicrostructure elements made of different relative amounts of basiccolors (as is the case with Multicolor Dithering) make the task ofreplacing individual document image elements by faked elements a veryhard task for potential counterfeiters.

Use of Special Inks such as Metallic and Iridescent Inks forStrengthening the Document Protection

Special inks such as metallic or iridescent inks offer an even strongerprotection against document counterfeiting attempts, since printingdevices with at least one print cartridge with a special ink are noteasily accessible to the general public. When observed from a givenviewing angle, a special ink may have one given color, whereas, whenseen from another angle, it may have a different color. This allows toembed a special ink in the parts of the target image specified by amask, which when seen by an observer from a certain angle yields nodifference with the surrounding parts and when seen from another angleconveys a distinct visual message, the message represented by the mask'sshape. One way to embed a special ink into its surrounding parts is tomeasure its spectrum with a spectrophotometer according to a givenmeasuring geometry, e.g. a collimated light source at 45 degrees and thelight sensor at zero degree (which is for example the geometry of theGretag SPM 500 spectrophotometer). From the measured spectrum, one mayobtain the corresponding CIE-XYZ values (the formula for converting aspectrum to a tri-stimulus CIE-XYZ value is given in the book: G.Wyszecki and W. S. Stiles, Color Science, 2nd edition, J. Wiley, 1982,pp. 155-158) characterizing the basic color of the special ink underthese viewing conditions. The basic color of the special ink is thenused for the color separation of the original image (see above thesection “Generation of microstructure images by Multicolor Dithering”,paragraph on color separation by tetrahedrization). Parts of an originalinput color image to be rendered with a special ink may be rendered by acombination of that special ink and of other basic colors, e.g. threeother basic colors. This technique allows to render an original imagecolor either with or without the special ink. When it is rendered with aspecial ink, the special ink is, at certain observation conditions (e.g.a certain viewing angle), hidden within the target image. At a differentobservation condition (e.g. at a different viewing angle), the partscovered by the special ink are revealed. As an example, FIG. 35B shows adocument seen from an angle where the parts covered by the special ink(e.g a metallic ink) reveal the message “TILT THE DOCUMENT, THIS PARTSHOULD DISAPPEAR”. The enlarged part of FIG. 35B clearly shows that thismessage incorporates the underlying microstructure, i.e. the underlyingmicrostructure is printed at least partly with the special ink.

In a similar manner, one may embed in a document an ultra-violet ink§which is hidden in the dithered image under normal viewing conditions(its tri-stimulus CIE-XYZ values, measured and computed as shown above,allow to embed the ultra-violet ink in dithered images). But, underultra-violet light, due to the fluorescence of ink under ultra-violetlight, the parts covered by the ultra-violet ink will be revealed, forexample: “THIS IS A VALID DOCUMENT”.

A similar behavior may also be expected from phosphorescent inks: undernormal viewing conditions, the phosphorescent ink is hidden in thedithered image (its tri-stimulus CIE-XYZ values, measured and computedas shown above, allow to embed the phosphorescent ink in ditheredimages). But, when put in the dark after exposure under light, the partscovered by the the phosphorescent ink will be revealed, for example,“THIS IS A VALID DOCUMENT”.

Use of Fluorescent Inks for Strengthening the Document Protection

Fluorescent inks can be used to offer a further level of protectionsince they are not available on standard desktop printers. Since theseinks tend to fade away, these inks may be used in security documentshaving a relatively short life time, for example travel documents,visas, airplane tickets or entrance tickets. The spectrum of afluorescent ink can be measured by a photospectrometer, converted into aCIE-XYZ value which is then used for color separation as explained inthe previous section “Use of special inks”. If the fluorescent ink isthe dominant ink, its fading effect may completely distroy themicrostructure and therefore considerably modify the global image. Thisallow to produce security documents with a limited life time.

Automatic Synthesis of a Dither Matrix

In many applications it is important to be able to generate the dithermatrix on the fly, preferably starting from a simple bitmap image (e.g.a black-white image, 1 bit/pixel) incorporating the microstructure'soriginal shapes. Such applications include the generation of images withsecurity features for use in security documents, which may need to becustomized and possibly personalized according to their content, i.e.their microstructure must vary depending on the content of the documentwhich is to be generated.

In addition several methods are proposed for equilibrating a ditheredimage, avoiding large spots with predominantly single color surfacessuch as white or black surfaces.

Symbols, logos, text and other pictorial elements can be represented asbilevel bitmaps. Bi-level bitmaps can also be obtained by scanningblack-white pictorial elements printed on paper.

Automatic generation of dither matrices from bitmap images relies on theapplication of morphological operators (see An introduction tomorphological image processing, by E. Dougherty, chap. 1, 3, pp. 3-18,66-75, SPIE Press, 1992). It also relies on re-ordering operations whichare applied to sets of successive pixels obtained during skeletonizationby morphological operators. The input bitmap can be of arbitrary size.Since the resulting dither array tiles the output image plane, theoperators are applied in a wrap-around manner. Coordinates of pixels arecomputed modulo the width and height of the bitmap. Various operatorsand combinations of operators as well as various re-ordering operationsare applied to the bitmap in order to generate the target dither array.

Shape Thinning for Obtaining the Foreground Dither Threshold Values.

The first part of the dither array generation method consists indetermining the cells which will contain the foreground dither thresholdvalues (cells with low values are set first when dithering the picture,they are usually part of the foreground of the shape). The preferred wayto achieve this is to apply a thinning algorithm (FIG. 17) on theoriginal bitmap and generate a list of pixel coordinates. In the presentembodiment, one cell in the dither array corresponds to one pixel in theinput bitmap. We use the thinning algorithm presented in Fundamentals ofDigital Image Processing, by Anil K. Jain, chap. 9, pp. 381-389,Prentice Hall, 1989, which yields connected arcs while being insensitiveto contour noise.

While applying the thinning algorithm to the bitmap, each thinning stepi provides a set Si of pixel coordinates. These pixels form the contourof the current shape, obtained by the previous thinning step; their setof coordinates is hereinafter called “contour pixel coordinates”. Thealgorithm stops when the bitmap skeleton is obtained. The skeleton isthe shape obtained when one further thinning step would have no effect(FIGS. 18A, 18B). The set of coordinates provided by one thinning stepSi is appended to an ordered list of sets L1 (FIG. 17).

The second part of the array generation consists in determining thecells which will contain the higher dither threshold values of thedither array (cells with high values compose the background of adithered picture). The corresponding pixels are usually part of thebackground of the initial bitmap image (e.g. the background of letter Ain FIG. 17). Many morphological operators, as well as combinations ofthem can be used to do so. We present two methods, both based on thedilation and thinning operators, the second method being applied to theinverse bitmap (video inverse), where black pixels become white andvice-versa. Hereinafter, we call the inverse bitmap “dual bitmap” (FIGS.19A, 19B).

To determine the higher dither values of the array, we couldrepetitively apply a dilation operator to the original bitmap.Morphological dilation allows to create new, bolder contours by growinga shape until it fills the entire bitmap space. However, little holeswithin the original bitmap are quickly filled while larger areas remainempty, blurring the contours of the microstructure shape after a fewdilation steps. With methods such as method I and II presented in thenext paragraphs, we constrain the dilation so that small gaps arepreserved, while larger empty spaces are used to grow the shape.

I. Alternated Dilation for Background Dither Array Values (FIG. 33A).

To compute the remaining array cells, we use the dual skeleton. The dualskeleton is obtained as the result of the thinning (iterative erosion)process applied to the dual bitmap (FIGS. 20A and 20B). We start thegrowing process with two patterns which are the initial bitmap (pattern1, FIG. 18A) and the dual skeleton (pattern 2, FIG. 20B).

At each step of this alternated dilation method, a dilation operator isapplied consecutively to pattern 1 (FIG. 21), then to pattern 2 (FIG.22). The dilation operator takes into account the result of the previousstep carried out on the opposite pattern: in each dilation step, newpixels are marked. If a particular dilation step tries to dilate a pixelmarked by a previous step (superimposed pixels), the dilation isignored. For example, when the pixel set by the dilation operatoroperating on pattern 1 is located on pattern 2, the pixel is not set. Wemaintain a set Sm of coordinates of the altered pixels in the patternsat each step m of the algorithm. Each of these sets is appended to anordered list of sets L2 (FIG. 22). For the two first steps, pixels partof the skeleton and dual skeleton are considered as the sets S0 and S1,located in the first and second place in the list L2. By construction,the content of each set Si is not ordered.

II. Dual Bitmap Thinning (Thinning of Background)

Another way to determine the position of the background dither arrayvalues is to use only the succession of steps occurring during dualbitmap thinning as a criterion (dual erosion). This corresponds to thesame process as was used to determine the foreground dither array values(lower values in array), except that the dual bitmap is given as inputto the algorithm, instead of the original bitmap itself (FIG. 23). Theresult of this operation is the same as with alternated dilation: weobtain an ordered list of sets L2, but the dither array shape growsdifferently. FIG. 33B shows an example where the background becomesdarker according to the succession of contour pixel coordinates obtainedby dual thinning. The few first contour pixel coordinates obtained bydual bitmap thinning are put at the end of list L2 in order to ensurethat the white outline around the initial bitmap microstructure shape(here an “A”) is darkened only at the highest darkness levels. Thisallows to preserve the microstructure shape also in very dark parts ofthe dithered image (90% darkness).

Merging Lists of Sets of Pixel Coordinates L1(Foreground) and L2(Background) into One List L

The two first parts of the array generation (the first part is shapethinning and the second part is either alternated dilation or dualbitmap thinning) provide two lists of sets L1 and L2, each setcontaining pixel coordinates. These lists can now be merged together bysimply appending the second list to the first one, resulting in a singlelist L. This ordered list of bitmap pixel coordinates is used forcreating the dither array, see section “Renumbering of dither cells”.More sophisticated merging operations can be realised. For example onemay equilibrate the distribution of black pixels in a tile byalternating the sets in the list L, one from L1, one from L2. In FIG. 34shows another example of creating list L″, where the discrete contourpixel coordinates lists Si′ associated to the background are obtained byalternated dilation. However they are inserted in a different order intolist L2 so as to obtain a shape growing from the background until itreaches the initial foreground bitmap shape (shape described by pixelcontours in list L1). Lists L2 and L1 are merged to form list L. Theparticular shape growing behavior shown in FIG. 34 ensures that themicrostructure shape remains apparent even at very dark levels (close to90% darkness).

Renumbering of Dither Cells

The last part of the dither array generation is the creation of a ditherarray of the size of the original bitmap and the numbering of the ditherarray cells according to the position of corresponding bitmap pixels inlist L. To avoid scan lines artefacts and ensure regular filling of thecontours, pixels from the same set Si are picked up in a random order.

Synthesizing an Equilibrated Dither Array by Combining a Low and a Highfrequency Dither Array

Since motifs (microstructure shapes) incorporated in large dither arraysmay not be well balanced, visually disturbing artefacts like alternatinglight and dark stripes may appear within the dithered image generatedwith a dither array obtained by the methods described above (FIG. 24).This phenomenon is accentuated by dot gain since middle and dark tonestend to become darker. In order to avoid such artefacts in the targetimage, it is important to equilibrate either the dither array or thefinal dithered image. Let us first describe one possible method forequilibrating the dither array based on the combination of the lowfrequency (LF) dither array synthesized from the initial bitmap and ahigh frequency (HF) dither array. The idea is to insert thehigh-frequency dither array in the background of the equilibrated ditherarray (FIG. 28). The term “high-frequency dither array” is used asgeneric term meaning that its embedded pattern is of significantlyhigher frequency than the microstructure embedded within the lowfrequency dither array.

In order to generate the equilibrated dither array, we first take thedither values of the L1 list corresponding to the foreground of thedither array. We then take the L2 list with the dither values of thebackground of the dither array. We remove from the L2 list one orseveral successive contours (e.g. pixel set Sp′ and Sp+1′) in order tocreate a clear separation between the foreground and the background ofthe microstructure shape. We associate to the sets of cells which havebeen removed from the L2 list (e.g. pixel set Sp′ and Sp+1′) the highestpossible threshold values yielding the background color even at a highforeground color intensity. In the case where the foreground is black orrespectively has a saturated basic color, this ensures that these cellsremain white even at a high darkness or respectively at high saturation.We then replace the remaining background cells (e.g. L2 minus theremoved pixel sets Sp′ and Sp+1′) with the content of a high frequencydither array. This high frequency dither array, for example the ditherarray disclosed in U.S. Pat. No. 5,438,431, and in the article (V.Ostromoukhov and R. D. Hersch, “Multi-Color and Artistic Dithering”,Siggraph'99, Computer Graphics Proceedings, Annual Conference Series,1999, pp. 425-432) comprises dither levels covering the full range ofdither values. For improved protection the high frequency dither arraymay also incorporate tiny shapes incorporating a 3rd level ofinformation such as symbols, characters or numbers (for example thegreek frize in FIG. 27, zoomed out on the bottom left).

The dither values of cells belonging to the foreground of the ditherarray (set L1) are numbered and scaled in order to also cover the fullintensity range or at least a significant part of it. In order to avoidscan lines artefacts and ensure regular filling of the contours, cellsbelonging to a same set Si are picked up randomly and given successivedither threshold values. FIG. 28 shows the resulting equilibrated ditherarray combining a low frequency dither array incorporating themicrostructure and a high-frequency dither array. FIG. 29A shows a wedgeand FIG. 29B a uniform intensity patch rendered with the equilibrateddither array.

When compared with the iterative equilibration technique described in V.Ostromoukhov, R. D. Hersch, “Multi-Color and Artistic Dithering”,Siggraph'99, Computer Graphics Proceedings, Annual Conference Series,1999, pp. 425-432, the presented method is much faster and moreaccurate, since it equilibrates the dither matrix specifically for theoriginal image. There is no need to apply the equilibration to a largeset of input intensity levels, neither to carry out several iterations.

Mixing a low-frequency dither array with a high-frequency dither arrayin this manner improves local equilibration, but also induces a globaltonal modification. In order to establish the reproduction curve usedfor tonal correction, one may print patches at different intensities,measure their density and deduce their surface coverage values, as isknown in the art.

An alternative means of improving the tone reproduction behaviorconsists in reassigning dither threshold values to the cells in the listL1 in such a way that for each intensity level to be reproduced, thenumber of added foreground pixels corresponds to the number of pixelsthat would have been added if the high-frequency dither array had beenused in the area covered by the microstructure shape. This number can beeasily computed by applying a mask corresponding to the foreground ofthe bitmap onto the high-frequency dither array and count the number ofpixels reproducing the desired foreground intensity level. By applyingthis procedure for consecutive discrete intensity levels, we selectsuccessive cells within successive sets of cells from list L1 (again bypicking each cell randomly within a single set Si) and assign to each ofthem a dither threshold level corresponding to the current discreteforeground intensity level.

Target Image Equilibration by Postprocessing

A second possible method for equilibration compensates the uneven localsurface coverage of the ink in the dithered picture by taking a portionof the foreground pixels (black) and redistributing it to the backgroundregions (white). It uses a high-frequency dither matrix to locate thepixels to be redistributed. High-frequency pixel redistribution takesinto account the dot gain and an approximation of the human visualsystem transfer function.

For this purpose we need to detect the regions in the dithered picturethat do not match accurately enough the intensity of the original image.As proposed by V. Ostromoukhov and R. D. Hersch, (in “Multi-Color andArtistic Dithering”, Siggraph'99, Computer Graphics Proceedings, AnnualConference Series, 1999, pp. 425-432), we simulate the dot gain byadding to each pixel the darkness or color intensity value representingthe dot grain of neighbouring pixels, e.g. horizontal and verticalneighbours contribute with a weight of 20% and diagonal neighbourscontribute with a weight of 5%. We then apply a Gaussian low-pass filterapproximating to some extent the low pass behaviour of the human visualsystem transfer function (HVS filter). The resulting filtered ditheredimage, hereinafter called “perceived dithered image” is compared withthe original image and the difference image, called “deltamap” is thenused for equilibrating the target image. The radius of the low-passfilter depends on the viewing distance and the resolution of thepicture.

Based on the estimation of about 30 cycles per degree for the cutofffrequency of the human visual system (Handbook of perception and humanperformance, L. Olzak, J. P. Thomas, chap. 7, pages 7-1 to 7-55, J.Wiley, 1986), we approximate the human visual system transfer function(hereinafter called “HVS filter”) by the Gaussian functionF(q)=Exp(−pq²), where the unit on the frequency axis (q-axis)corresponds to the cutoff frequency of 30 cycles per degree. Thecorresponding impulse response, i.e. the inverse Fourier Transform ofF(q), is also a Gaussian function, f(r)=Exp(−pr²), whose unit (r-axis)corresponds to {fraction (1/30)} degree of visual angle. To produce thediscrete convolution kernel, the Gaussian impulse response function issampled on a 5s×5s grid, where the standard deviation s=1/Sqrt(2p). Fordifferent printing resolutions as well as for different observationdistances (e.g. for posters to be observed from far away) the discreteconvolution kernel needs to be recomputed accordingly.

For example, at 1200 pixels per inch and at an observation distance of25 inches the visual angle formed by one inch is in degrees a=({fraction(1/25)}*360/2p). A visual angle of {fraction (1/30)} of degrees, wherescreen element details should disappear, corresponds to(1200/a)*({fraction (1/30)})=17.45 pixels and s=1/Sqrt(2p) correspondson our pixel grid to 17.45/Sqrt(2p)=7 pixels. A convolution kernel ofsize 5s×5s corresponds in this example to a kernel of size 35×35 pixels.

After applying dot gain simulation, human visual system filtering andcomparison between the original and the perceived dithered image, weobtain a delta map Dm(x,y), composed of the pixel by pixel intensitydifferences between the initial input image P(x,y) and the perceiveddithered image H′(x,y) (what is “seen”). Negative deltas indicate thatthe dithered picture is “seen” too bright locally, while positive deltasindicate that it is “seen” too dark. For convenience, the deltamap iscomputed as 2's complement 8 bit numbers. FIG. 25 shows a schematic viewof the steps necessary to obtain the delta map. In the resulting printeddeltamap, positive values are expressed by dark intensity levels(black=0 means no change, 1 means add 1, etc . . . ) and negative valuesare expressed by high intensity levels (white=255 means subtract 1, 254means subtract 2, etc . . . on a 256 intensity level range).

We need to add a number of black pixels in the dithered image tocompensate for a too high brightness, and remove a number of blackpixels where the picture is seen too dark. In our delta map, positivevalues can be seen as the proportion of white to be added to black areasto reach the desired local gray level. Negative values represent theproportion of white to be removed from white areas.

The delta map Dm(x,y) is dithered with a high frequency dither arrayresulting in a dithered deltamap Dmd(x,y). This dithered deltamapDmd(x,y) is composed with the dithered image H(x,y) as follows. In areaswhere the delta map is positive, i.e. in black areas where black pixelsneed to be removed, the dithered deltamap Dmd(x,y) is ORed with thedithered image H(x,y). New white pixels will appear in the black partsof the dithered image. In areas where the delta map Dm(x,y) is negative,i.e. in white areas where black pixels need to be added, the dithereddeltamap Dmd(x,y) is ANDed with the dithered image. H(x,y) yielding thefinal equilibrated dithered image Q(x,y). New black pixels will appearin the white parts of the dithered image.

In order words, as shown in FIG. 26, in a preferred embodiment thefollowing logical operations are performed:

-   -   Dm(x,y)=P(x,y)−H′(x,y), where the minus is the 2's complement        minus on 8 bit values    -   If H(x,y)=0 (black), Q(x,y)=H(x,y) OR Dmd(x,y);    -   If H(x,y)=1 (white), Q(x,y)=H(x,y) AND Dmd(x,y).

To provide adequate equilibration, the high frequency pattern present inthe high-frequency dither array needs to be several times smaller thanthe low frequency pattern. Any dither array comprising very smallclusters may be used. In the example shown in FIG. 32 (original in FIG.30, dithered with only the low-frequency dither matrix in FIG. 31), weuse as high frequency dither matrix the rotated dispersed dither matrixproposed by V. Ostromoukhov, R. D. Hersch and I. Amidror (“RotatedDispersed Dither: a New Technique for Digital Halftoning”, Siggraph'94,Computer Graphics Proceedings, Annual Conference Series, pp. 123-130,1994) since it exhibits a semi-clustering behaviour at mid-tones. It istherefore less sensible to dot gain than dispersed-dot halftones. Thehigh-frequency dither array may also incorporate a second levelmicrostructure made of artistic patterns or tiny shapes such as symbols,characters or numbers (greek frize in FIG. 27).

It is important that the dot gain of the high-frequency dither array becorrectly compensated. We can establish its tone reproduction behaviorby printing a series of halftoned patches for different gray levels andmeasure their density. Using the Murray-Davis formula (H. R. Kang, ColorTechnology for Electronic Imaging Devices, SPIE Optical EngineeringPress, 1997,section 2.2: Murray-Davis equation, pp 42-43), we determinethe actual proportion of black on paper for each patch and compute thetone reproduction curve. During the equilibration process, the tonereproduction curve is used in order to compute for the deltamap valuesDm(x,y) tone-corrected deltamap values Dm′(x,y) which are dithered toyield the dithered deltamap Dmd(x,y). Equilibration by postprocessing iscarried out in a single pass and is specific to the desired targetimage. It is therefore faster and more accurate than the iterativeequilibration technique described in V. Ostromoukhov, R. D. Hersch,“Multi-Color and Artistic Dithering”, Siggraph'99, Computer GraphicsProceedings, Annual Conference Series, 1999, pp. 425-432.

Automatic Production of Security Documents

It is possible to run a computer program operable for creating anoriginal document image according to information related to saiddocument, such as for example the type of the document, the name of thedocument holder, the issuing institution, the validity of the document,the background to be inserted into the document, etc . . . .Furthermore, a slightly different computer program may alsoautomatically generate the bitmap incorporating the microstructureshapes by inserting text or graphics into a bi-level bitmap according todocument related information. These computer programs may carry outoperating system calls in order to embed text, graphics and images intoa document image, respectively a bitmap and save that document image orrespectively bitmap as a file on the computer running the program.

Such computer programs can be embedded into a preparation softwaremodule capable of generating both the original document image and thebitmap incorporating the microstructure shapes according to theinformation related to the target document to be created.

With such a preparation software module, a complete automatic securitydocument production chain may be established: upon a specification of asecurity document by document related information the following stepsallow to generate a security document:

-   -   (a) producing an original document image comprising said        document related information;    -   (b) producing a bitmap incorporating microstructure shapes        expressing said document related information;    -   (c) synthesizing a dither array with said bitmap;    -   (d) dithering the original document image with the synthesized        dither array, thereby generating the security document, where        both the global document level and the microstructure level        incorporate document related information.    -   (e) equilibrating the dithered original image thereby producing        the target security document

Step (e) is optional and applied for improving the quality of theresulting target security document. The generated security documents arefully personalized, since both the original document image and themicrostructure incorporate the document related information (e.g. thedocument shown in FIG. 36).

Generation of Security Documents via a Global Communications Network

Referring to FIG. 42, a web-based server system 2 for generatingsecurity documents such as commercial instrument printable files, isaccessible via a global communications network such as the internet 4 bya user or a customer at a client site 6, having a printer 8 and apersonal computer 10 or other computing means connected to thecommunications network 4. The server system may be a distributed systemcomprising servers or other data processing systems or databases at asingle site, or at different sites interconnected with a communicationsnetwork such as the internet, an intranet, or a local area network. Theweb-based server system 2 comprises a web server 12 including, orconnected to, a customer database 14 in which information on customersis stored, a payment server or system 16, for example for effectingcredit card payments, bank transfers and the like, and a productionserver 18 for performing calculations and other operations to createticket images and package data files for transmission and printing. Theproduction server may be interconnected to a context database 20 forstoring background images and other information concerning thecommercial instrument.

It will be understood by skilled persons in the art that theconfiguration of the above described server system may be modifiedwithout departing from the scope of this invention, the various serversand databases being depicted merely as examples in order to understandthe function of a possible server system for generating printedcommercial instruments according to this invention.

The server system for creating the commercial instrument may also be aproprietary system or an enterprise server system as illustrated in FIG.44, whereby the user accesses the enterprise server system 2′ through alocal area network or direct connection from a terminal 10′. In thisconfiguration, a user would typically be the issuer of the commercialinstrument and the payment transaction would occur between the purchaserof the commercial instrument and the user.

Referring to FIG. 45, a local or stand-alone server system 2″ is shownincorporating in a single data processing system the functions of theenterprise server system 2′ of FIG. 44.

Referring to FIG. 43, a flow-chart generally illustrating the generationof a commercial instrument, such as a printed ticket, with a dataprocessing system such as the server system described above, is shown.Initial operations include connection of the customer or user to theserver system 2, 2′, 2″, and subsequently selection and specification ofthe product. For example, if the commercial instrument is atransportation ticket, the customer may specify the journey departureplace and destination, the travel date and/or time, the class, the seat,etc. The initial steps may also comprise an identification procedure,particularly if customer information from a customer database is to beretrieved for inclusion in the printed commercial instrument, in whichcase the identification procedure may be after or before productselection and specification. An identification procedure may also berequired where commercial instruments are issued only to known oridentified persons. The term “product” shall be understood herein togenerally mean the event, service, or item being purchased or transactedto which the commercial instrument relates.

Once the product has been selected and specified, a payment order iscreated, for example using credit card, bank transfer or cash cardinformation supplied by the customer to the server system which thenlogs the payment order and/or sends a provisional payment order to apayment system 16. The transaction amount will not be debited accordingto the payment order until confirmation that the ticket has been sent tothe customer site.

After product selection and specification in step 24, a ticket imageprinting file is generated in the production server 18 using informationreceived from the web server portion 12 and, as the case may be, thecustomer database 14 and the context database 20, such that productinformation, personal information and contextual information may beincluded in the ticket image generation process.

It may be noted that ticket image generation may be performed inparallel, before or after generation of the provisional payment order(step 28). The ticket image file is then packaged and preferablycompressed such that it can be efficiently transmitted over acommunications network such as the internet, and printed on a standardPC printer. The ticket image may for example be received on the user'sor customer's computer screen as a page displayed in a web browser, orby e-mail, for example in commonly used text and image formats such asPDF, GIF, PNG, and the like that enable printing on personal PC printerswith the appropriate PC software. The sending of the commercialinstrument image file to the user's or customer's computer or terminalalso generates a confirmation to execute the payment order in waiting.

It will be apparent from the above that the security of the commercialinstrument does not reside in the inability to print or copy numeroustickets, since the customer receives the printing file, or could simplycopy a printed instrument. Security against the use of multiple copiesis provided by personal or unique information. For example, for atransportation ticket, the date, destination and a photographic portraitof the bearer of the instrument will make the commercial instrumentunusable by other persons and usable only during the period of validityby the bearer. For entertainment events, such as theatre, sports,cinema, or similar events, it would not always be necessary to includepersonal information if for example unique information such as a seatnumber, in conjunction with a date and time or venue, would be includedin the ticket image.

In FIG. 46, a flow-chart illustrating various steps or operations in thegeneration of a ticket image according to this invention is shown.

In the specific example illustrated in FIG. 46, a train ticket for thejourney from Milano Central Station to Berlin Zoological Garden, insecond class, and valid on a specific day, is described. The productinformation 38 will have been specified by the user, who will also haveprovided customer information 40 that enables his/her identification andretrieval of further customer information from the customer database 14.The customer database may for example include a library of photographicportraits of customers. On the basis of the customer identity, thecorresponding portrait of the customer is retrieved from the customerdatabase for inclusion in the ticket image, as will be described furtheron.

The product information 38 is sent to the production server orproduction server portion of the server system and used to select abackground image 44 from the context database 20 and a microstructurepattern or shape 46 that will be applied to microstructure elements 48in a mapping procedure 50, for example a planar mapping procedure. Themicrostructure elements 48 are organized so as to provide information,in particular information comprising text and numbers relating to theproduct information, for example in the case of a train ticket thatindicates the journey starting and destination places, the date, andpossibly additional information such as the class, the price and anyother product specific information. It is also possible to include inthe microstructure elements customer information such as the customer'sname, address, date of birth or other information specific to thecustomer. The microstructure elements may also comprise elements havingvarious graphical shapes or logos.

The product information is also used by the production server or serverportion to generate a product information layer that may include asimple presentation of the product specific information, in the presentexample of a transportation ticket, relating to the starting place anddestination, the class, the price, the validity, date or period,possibly further including electronically verifiable security featuressuch as an encrypted number code or a bar code. The coded orelectronically verifiable security feature provides additionalverification means for a person controlling the authenticity of theticket in case there is any doubt after a visual verification, or forany other reason, such as arbitrary spot checks.

The background image may be a photographic image, a drawing, or anyother image that is preferably non-uniform and representing places,objects, events, or other things that may be easily recognized andinterpreted visually, in other words, images that have some meaning orhave characteristic features that facilitate the memorizing andrecognition thereof by a person verifying the authenticity of theinstrument. The background image is preferably an image that isproprietary and not easily available to the general public. Thebackground image may be changed on a regular basis in order to increasethe difficulty of reproducing the ticket image. Where the commercialinstrument comprises a portrait 42 of the customer, the background andportrait images may be merged by any standard merging technique or bysuperposition 45 of the photograph on the background image to form apersonalized contextual image layer 54.

The microstructure pattern or shape 46 is for example a mathematicalimage deformation algorithm (warping transformation) as as describedhereinabove or as used in planar mapping or other known imagedeformation techniques. The microstructure shape or pattern may varybetween different types of commercial instruments on criteriaestablished by the issuer, for example, different shapes for differentticket values, events, days of the week, months, etc. The shape orpattern may also be regularly changed, for example but not necessarily,when the background image is changed, in order to make reproduction offorgeries more difficult by reducing the time during which a backgroundimage and a pattern on the image remain valid.

The microstructure elements 48 include in a preferred embodimentalphanumerical characters that enable product specific information suchas the date or period of validity, the event, seating number,information on the journey starting place and destination, and the liketo be read. The text that identifies the specific purpose of thecommercial instrument may in itself be unique (such as a combination ofthe title, date and seating number of a theatre event) or may be uniquein conjunction with customer specific information (for example a trainticket indicating a combination of a date, a journey, and a portrait ofthe traveller). The microstructure elements are used to create a dithermatrix representative of a microstructure image layer 56 that isrendered with the contextual image layer 54.

The microstructure elements used in the present application, whichinclude alphanumerical characters, are generated at a size that enabletheir reading at a personal document reading distance which maytypically be in the order of 20 to 50 cm from the eye. Themicrostructure elements are thus significantly larger than the screendot sizes provided in even the lowest resolution printed imagestypically available.

The microstructure elements are advantageously generated by automaticsynthesis from bi-level bitmap elements as already describedhereinabove, however the production of commercial instruments or otherdocuments with security features according to this invention may usemicrostructure elements generated in other manners, as described forexample in relation to FIGS. 50 a to 50 g which show various graphicalrepresentations of a microstructure element.

An Alternate Way of Synthesizing Microstructure Elements

The microstructure element 48 may be represented as a three-dimensionalelement 61 against a background 63 as shown in FIG. 50 a, whereby thedepth of the element, in the direction coming out of the paper of FIG.50 a, may be separated into a plurality of planes parallel to the paper,each plane defining a grey level. The microstructure element may forexample be defined in 256 planes that correspond to 256 grey levels,which equates with the number of grey levels commonly defined instandard printing techniques. The background 63 comprises “noise” thatcan be graphically represented as randomly distributed “peaks” that,when intersected by the high grey level planes, give the background agrainy aspect as shown in FIGS. 50 b to 50 d. When the grey level isvery high, the character will be at its thickest with a dark background,as illustrated in FIG. 50 b, the background getting lighter as the greylevel decreases as represented successively by FIGS. 50 c to 50 e. Foran intermediate grey level, the character is of medium thickness asshown in FIG. 50 f, or if there is a very low grey level, the characteris very thin as shown in FIG. 50 g. In this example, grey levels arethus varied by adjusting the thickness of the characters, in addition tovarying the density of the grainy background for the high grey levels,whereby it should be noted that the characters are preferablyhormomorphic such that, as they reduce in thickness, their general shaperemains. The latter property ensures the readability of the characters,whether depicting a low grey level or a high grey level.

Instead of adjusting the thickness, other techniques are available fordefining the microstructure element grey level, for example thecharacter may be defined by a dark border of a constant outer shape anddimensions while varying border thickness towards the center of thecharacter depending on the grey level.

The representation of lettering as microstructure elements is forexample depicted in FIG. 47, whereby the lettering on the left side ofthe figure are simple characters and on the right side of the figureshown as three-dimensional microstructure elements that graphicallyrepresent the microstructure dither matrix. In FIG. 47, the elementshave already been subjected to a planar mapping procedure 50 with amicrostructure shape (which in the specific example emulates thepositioning of text lines around a cylinder). It should be noted thatthe 3D representations of FIG. 50 a and on the right side of FIG. 47 aremerely means of assisting the reader in obtaining a visualinterpretation of microstructure elements which are in fact defined in adither matrix and could be represented in other ways.

In generating the dither matrix, account is taken of both the text ofmicrostructure elements and the microstructure shape. Moreover, themicrostructure image is scaled to the same size as the contextual image.The contextual image and microstructure images 54, 56 are then renderedby applying the microstructure dither matrix to the contextual imagewith any of the dithering methods described herein.

By way of example, one simple method of fusing is standard halftoning,as described above.

The results of standard halftoning is illustrated for example in FIG.48, whereby the contextual image and microstructure image are subjectedto the above described standard halftoning procedure resulting in ahalftoned image 58. As may be noticed in this halftoned image, in thelight areas of the contextual image, the microstructure characters arevery thin (because of the low grey level value) and in the dark areasvery thick (because of the high grey level value).

The grey level values of the dither matrix located betweenmicrostructure elements are preferably set at a low grey level valuevarying randomly such that the shape of the microstructure elementremains visible (even in dark areas) after the halftoning process.

It may be further noticed that the thickness of the microstructurecharacters vary along portions thereof, depending on the grey level ofthe contextual image in the vicinity of the portion of character inquestion.

Referring to FIG. 53, the effect of applying a microstructure dithermatrix of a microstructure element 48′ to an image 44′ by a halftoningtechnique is illustrated. This halftoned image 58 shows the varyingthickness of the character “T” 61 and the density of the backgroundgrain 63 as a function of the grey level of the image 44.

The visual quality of the computed halftoned image 58 is often notoptimal due to the size of the microstructure elements composing thedither matrix. In order to improve the quality, the rendering proceduremay further include an equilibration procedure as already describedherein. Other equilibration or balancing techniques, for example whichcompare the halftoned image with the contextual image as illustrated inFIG. 49, may be used. A balancing or equilibration technique that may beemployed includes examining the neighbourhood of each point of thehalftoned image, subsequently counting the number of black points andwhite points which are then used to compute an average grey level value,for example the number of white points divided by the total number ofpoints in the considered neighbourhood. These average grey level valuesare compared with the grey level value of the corresponding point of thecontextual image and if the difference is small, (for example below adefined or approximated value), then it is considered that the halftonedimage is a good approximation of the contextual image at that point. Ifthe difference between the compared grey level values is large, then itis considered that the halftoned image is locally a bad approximation ofthe contextual image and that the considered point of the halftonedimage should be inverted, in other words, set to white if originallyblack, or set to black if originally white. A probalistic function maybe used to determine whether the difference between the compared greylevel values is to be considered small or large.

While the halftoning and balancing or equilibration procedures have beendescribed as separate procedures hereinabove, it would be possible tocombine these two procedures in a single procedure, even if the terms“halftoning” and “balancing” are used separately.

A simple graphical product information layer 52 may be superposed 61 orotherwise merged with the rendered image 60 to result in the completedcommercial instrument image 62, such as the sporting event ticket imageillustrated in FIG. 52. As can be seen in FIG. 52, the simple productinformation layer 64 indicates the validity date, the event name, andthe price in an easy to read format, at least a part of this productinformation also being present in the microstructure of the renderedimage and easily readable at a normal document reading distance of say20 to 50 cm from the eye. In this particular example, the microstructurelayer also comprises the name of the sporting event attendee. At thesame time, the contextual image which in this example includes aphotograph of a trophy and a person's portrait, is well-defined atnormal document reading distance, and even improves beyond the documentreading distance, say at arms length from the eye where themicrostructure characters become less apparent. Visual verification of acommercial instrument generated according to this invention may includethe steps shown in FIG. 51, whereby a ticket controller would check therelevance of the ticket information by reading the product informationlayer 64, and the microstructure information, which should correspond tothe product information. The controller may also verify the contextualimage and the microstructure shape or pattern and in this regard shouldbe informed of the background image and microstructure pattern thatapplies to the type of commercial instrument at its date of validity. Abar code 66 which preferably includes an encrypted code, may be used asan additional verification means in case of doubt of the authenticity ofthe ticket, or for other reasons, such as random checks.

Referring to FIG. 52 a, a detailed view of a portion of the printedticket of FIG. 52 is shown. The smallest screen dots used for printingthe image are conventional screen dots using traditional shapes, such asan ellipse or circle. The screen dots may however be provided with aspecial shape that could be changed on a regular basis to increase thesecurity against forgery. This security technique may be taken furtherby introducing additional layers of microstructure elements having sizesintermediate the smallest printed screen dot and the microstructureelements verifiable at normal reading distance, using a renderingprocedure as set forth above. Verification of the intermediatemicrostructure elements may be performed by close visual inspection ofthe printed instrument, for example at a distance of 10 to 20 cm fromthe eye. As best seen in FIGS. 52 a and 52 b, in this example a secondlayer of smaller microstructure elements comprising the characters“05-05-04” is provided, indicating the date of the event. In otherwords, a plurality of microstructure image layers formed bymicrostructure elements of a different size for each layer could berendered with the contextual image with the rendering proceduresdescribed above.

It may be noted that commercial instruments or other security documentsgenerated through a communications network as described above may alsobe displayed on an electronic screen, for example the screen of aportable electronic device, rather than being printed.

Distinctive Features and Document Protection Features

The present invention thus protects security documents comprisingelements such as text, a photograph, graphics, images, and possibly abackground motive by incorporating microstructures having shapes varyingwith the intensity of the document elements. Since, thanks to thedithering process, the target document image is built on top ofmicrostructures, both document elements and microstructures cannot beerased or modified without significantly modifying the target documentimage. For example in FIG. 27, one can see that in this example, all theelements making up the image are microstructures. The global image isthe girl's face. The first level microstructure is a dragon. Thehigh-frequency dither array incorporates a second level microstructurein the shape of a greek graphic symbol (a frieze). Such a second levelmicrostructure can incorporate simple second level microstructure shapessuch as one or a few letters, numbers or symbols for additionalprotection. This second level microstructure embedded into thehigh-frequency dither array makes it even harder to create fakeddocument images or document elements.

A key distinctive feature which characterizes the present invention isits ability to synthesize the microstructure in the form of a dithermatrix starting from a bilevel bitmap incorporating the microstructureshape, the generated dither matrix being sufficiently sophisticated formaking the chosen microstructure visible both at high and low imageintensities. For example in FIGS. 33A and 33B, the microstructure isvisible at a darkness of below 10% and higher than 90%. The hebrewletters in FIG. 34 are clearly visible between 10% and 90% darkness.Furthermore, the synthesis of the dither matrix can be carried outautomatically by a computer program.

A second distinctive feature of the present invention is its ability tocreate geometrically transformed microstructures allowing to createvariations of the security document, while keeping the global imageintact and without modifying the information (e.g. text) carried at theglobal level and at the microstructure level. These geometricallytransformed microstructures also allow to generate on a display animage, whose microstructure is animated. For example, FIG. 6 showsseveral instances of the same image and the same microstructuregenerated with different transformation parameters.

A third distinctive feature of the invention is its ability to carry outequilibration by making use of a high frequency dither matrix, possiblyincorporating a second level microstructure (FIG. 27).

A fourth distinctive feature is the possibility of generating colordocuments with standard, non-standard and special inks, where one,several or all contributing inks are part of the microstructure.Considered inks are for example metallic, iridescent, fluorescent,phosphorescent and ultra-violet inks.

A fifth distinctive feature of the present invention is its ability ofautomatically synthesizing personalized security documents frominformation related to the document content.

Let us enumerate the main protective features. A first protection isensured by the continuity of the microstructure when crossing adjacentelement boundaries (pieces of text, graphic elements, images). Thiscontinuity makes it extremely hard for potential counterfeiters toreplace given document elements by faked elements (for example replace aphotograph by a faked photograph). As a second protective feature, text,represented in the original image as dark typographic characters can beprotected by the microstructure. A third protection is offered by thedithering process used for microstructure image synthesis which ensuresthat the microstructure shape thickness varies according to the currentimage intensity or when colors are used, according to the dominant colorintensities (or ink coverage). Counterfeiters cannot therefore simplyincrust into a document by alpha blending a pseudo microstructuregenerated with standard desktop graphic packages. A fourth protection isoffered by allowing text to be part of the microstructure, providingadditional means of verifying the authenticity of the document. Thisallows to establish a correlation between information at the globaldocument level and at the microstructure level. For example, the name ofa document holder may be repeated all over the document by embedding itinto a microstructure made of text (at the first or possibly at thesecond microstructure level). Modifying that name would require tomodify the microstructure warped over all the document, an almostimpossible task. A fifth protection is offered by the possibility ofgenerating different instances of the microstructure on differentdocuments using the parametrized transformation T_(t)(u,v) and possiblythe warping transform T_(w)(x,y). A given instance of the microstructureimage defined by a particular parametrized transformation T_(t)(u,v) maybe correlated with the document content, for example the value of thesecurity document, the type of the document and the year when thedocument is issued.

FIG. 36 shows as an example of a security document a diplomaincorporating a microstructure containing the name of the documentholder and the name of the institution issuing the diploma. Since themicrostructure covers all document parts, parts of it cannot bereplaced. Furthermore, thanks to the geometric transformation whichwarps the microstructure across the picture at different orientationsand sizes, and thanks to the fact that the thickness of themicrostructure adapts itself to the local image intensity,microstructure elements cannot be simply copied from one location tomany other locations. In addition, in dark (or color saturated) parts ofthe document, the very thin separations between microstructure shapesmake the the unauthorized document reproduction very difficult.

Creating Security Documents with Microstructures Incorporating SpecialInks

The document protection by microstructures is not limited to documentsprinted with black-white or standard color inks (cyan, magenta, yellowand possibly black). According to pending U.S. patent application Ser.No. 09/477,544 (Method an apparatus for generating digital halftoneimages by multi-color dithering, inventors V. Ostromoukhov, R. D.Hersch, filed Jan. 4, 2000, due assignee: EPFL), it is possible, withmulticolor dithering, to use special inks such as non-standard colorinks, metallic inks, fluorescent or iridescent inks (variable colorinks) for generating security documents. In the case of metallic inksfor example, when seen at a certain viewing angle, the microstructureappears as if it would have been printed with normal inks and at anotherviewing angle, due to specular reflection, the microstructure appearsmuch more strongly. A similar variation of the appearance of themicrostructure can be attained with iridescent inks. Such variations inthe appearance of the microstructure completely disappear when theoriginal document is either scanned and reproduced or photocopied.

Furthermore, one may incorporate non-standard inks only in certain partsof the security document and print the other parts with standard inks.Then, the effect of a metallic ink may only be visible within documentparts selected by a mask, the mask itself being capable of representinga visual message such as a text, graphic symbols, a graphic design or adithered image. For example, one may use as a mask the dragon of FIG. 27and render within the target image with metallic ink only those parts ofthe microstructure which are covered by the dragon shape (the dragonshape is obtained by simple dithering of the original image, withoutequilibration). In such a target image the dragon shape is highlightedby the metallic ink, when seen at an angle allowing specular reflectionof the incident light.

Creating Security Documents on Screens or Supports Other Than Paper

Document images incorporating microstructures may be used to generatesecurity documents non only on paper but also on electronic displays(e.g. computer or mobile phone screens) or on other supports such astransparent or opaque plastic material, polymer material, packages ofvaluable products, optical disks such as CD-ROMs or DVDs, or in opticaldevices such as diffractive elements, holograms and kinegrams.

Creation of Artistic Images by Automatic Synthesis of the Microstructure

The automatic synthesis of microstructure images opens very efficientways for designing artistic images such as illustrations, posters andpublicity images. The designer only needs to create an original imageand original microstructure shapes. With the help of a standard desktopgraphic package, he can scan the microstructure shapes or draw them,retouch them so as to meet his aesthetic wishes and convert them into anoriginal microstructure bitmap needed for the automatic synthesis of thecorresponding dither matrix. This dither matrix incorporating themicrostructure shapes is then used to dither the original image andproduce the target artistic dithered image. Therefore, once integratedinto a desktop software package, automatic dithering is a very effectivetool for creating graphic designs, posters and publicity. In addition,large scale posters may be created easily where from far away the globalimage is visible and from nearby the microstructure becomes visible.This microstructure incorporates a second layer of information such astext, logos, a graphic design or publicity. Such large scale posters arespecially effective when situated for example on highways, where cardrivers see at first the global image and then, when coming closer theysee microstructure information.

Creation of Images with Animated Microstructures

Images comprising animated microstructures can be used to createbeautiful information and publicity sites attracting the attention ofclients. Especially for clients visiting Web sites, images with animatedmicrostructures are capable of forwarding a message incorporated intothe animated microstructure. Parent patent application U.S. Ser. No.09/902,227, filed Jul. 11, 2001, by R. D. Hersch and B. Wittwerdiscloses a method for generating animated microstructure images, i.e.image sequences and animations, where from where from far away mainlythe image is visible and from nearby mainly the evolving microstructureis visible. That method makes use of a large dither matrix incorporatingthe microstructure. Microstructure evolution is obtained by successivelyregenerating new instances of the image with modified transformationparameters. Thanks to the method for the automatic synthesis of dithermatrices disclosed in the present invention, aesthetic dither matricescan be easily and rapidly produced and hence greatly facilitate thecreation of images with animated microstructures.

The disclosed methods have been described with respect to particularillustrative embodiments. It is to be understood that the invention isnot limited to the above described embodiment and that various changesand modifications may be made by people skilled in the art withoutdeparting from the spirit and scope of the appended claims.

Computing System for Synthesizing Security Documents and MicrostructureImages

A computing system (FIG. 37) for synthesizing security documentscomprises an interface for receiving a request for generating a securitydocument, for example the diploma shown in FIG. 36. Relevant information(370, FIG. 37) is received with that request for example the name of thedocument holder, the issue date and the type of document to be issued.The computing system also comprises a preparation software moduleoperable for preparing the data used for the production of the securitydocument and a production software module operable for producing saidsecurity document. The preparation software module running on thecomputing system may generate the original document image, themicrostructure shapes and possibly transformation parameters accordingto information received together with the request. The productionsoftware module first synthesizes the microstructure to be used forgenerating the security document and then synthesizes the securitydocument with that microstructure which is then transmitted to an outputdevice.

In a preferred embodiment (FIG. 37, terms in parenthesis), themicrostructure shapes are generated by producing a bitmap incorporatingthe microstructure shapes. The microstructure to be used for generatingthe security document is embodied in a dither array which is synthesizedfrom said bitmap by applying to the bitmap mathematical morphologyoperations. Synthesizing the security document is carried out bydithering the original document image with the previously synthesizeddither array.

A similar computing system (FIG. 38) can be operated for synthesizingmicrostructure images such as microstructure images for graphic designs,information, publicity and posters. The computing system comprises aninterface operable for receiving an original image, microstructureshapes, possibly a transformation selected from the set of availabletransformations and transformation parameters, as well as, in the caseof color, a selection of the basic colors to be used for rendering thetarget dithered image (380, FIG. 38). The computing system alsocomprises a production software module operable for producing saidartistic microstructure image. The production software module running onthe computing system takes as input the microstructure shapes,synthesizes the microstructure, and produces the target microstructureimage incorporating the microstructure.

In a preferred embodiment, the microstructure shapes are incorporatedinto a bitmap received by the computer system's interface. Themicrostructure to be used for generating the security document isembodied in a dither array which is synthesized by the productionsoftware module from said bitmap by applying to the bitmap mathematicalmorphology operations. Synthesizing the target microstructure image iscarried out by dithering the original document image with the previouslysynthesized dither array and if in color, possibly according tospecified basic colors, and possibly according to the transformation andtransformation parameters received by the computing system's interface.

Computing System for Displaying Images with Animated Microstructure

Images with animated microstructures can be synthesized offline by acomputer running an animated microstructure image rendering software.The resulting image animation can be then incorporated into Web pages asanimated images (e.g. animated GIF or MNG formats). An alternativeconsists in creating an image computing and display system, for examplean applet, running the animated microstructure image rendering software.In that case, the image computing and display system will run on theclient's computer and display the animated microstructure image or imageanimation. As a preferred embodiment, the image computing and displaysystem will receive from the server computing system (FIG. 39) as inputdata the input color image, the dither matrix, the animationtransformation, the warping transformation, the set of basic colors{C_(l)} and a possible mask layer. With the present technology, thepreferred embodiment of an image computing and display system is a Javaapplet. The image computing and display system's program (e.g. theprogram running as an applet) will then generate and display the targetimage by carrying out the initialization, image rendering and imagedisplay steps described above.

In addition, specific embodiments of the animated microstructure imagerendering system may allow to tune some of the image renderingparameters according to user preferences or user profiles. For exampleone image selected from a set of images, one set of basic colorsselected from various sets of basic colors, one dither matrix selectedfrom different dither matrices, one animation transformation andpossibly a warping transformation may be tuned according to userpreferences or profiles. These specific embodiments allow to customizethe animated microstructure images according to users or usercategories.

Optionally, a specific server (e.g. a Web site) can be conceived whichallows designers to create images with microstructures evolving overtime (i.e. animated microstructure images) on their own computers (FIG.40). The program interface running on their computers (e.g. dynamic Webpage incorporating an applet) will exchange information with the server.With such a Web based design interface, graphic designers may specify orcreate the source image, the dither matrix, the basic colors, theanimation transform, the warping transform and the image mask layer. Bybeing able to modify interactively each of these parameters andelements, and immediately visualizing the results, designers may be ableto interactively create appealing images with animated microstructures.Upon signing a licensing-agreement, they may then receive theauthorization to transfer the animated microstructure rendering software(e.g. the applet's code) as well as the created data elements into theirown Web pages. FIG. 41 shows an animated microstructure imageincorporated into a Web page.

1-40. (canceled).
 41. Method of generating an image incorporating amicrostructure, including obtaining an original image; generating amicrostructure; and rendering a region or the whole said original imagewith said microstructure; wherein the operation of generating themicrostructure includes an automatic synthesis of microstructureelements as a microstructure dither matrix from original microstructureshapes.
 42. Method according to claim 41 wherein the microstructureincludes a low frequency microstructure with low frequencymicrostructure elements generated from the original microstructureshapes, and a high frequency microstructure with high frequencymicrostructure elements, whereby the low frequency microstructureelements are larger than the high frequency microstructure elements. 43.Method according to claim 41 wherein the synthesis of the dither matrixincludes applying mathematical morphology operators to themicrostructure shapes.
 44. Method according to claim 43 wherein theapplied mathematical morphology operators comprise a shape thinningoperator for the bitmap shape foreground and an operator selected from aset of alternated dilation and dual bitmap thinning for the bitmap shapebackground.
 45. Method according to claim 41 wherein the synthesizedmicrostructure elements are visible at both high and low intensitiesafter rendering with the original image.
 46. Method according to claim41 wherein the original microstructure shapes are bitmap elements. 47.Method according to claim 41 wherein the visibility of themicrostructure elements is tuned by a mask whose values representrelative weights of the original image halftoned with conventionalmethods and the original image synthesized with the microstructure. 48.Method according to claim 41 further including applying a parametrizedtransformation to warp the microstructure incorporated in the image. 49.Method according to claim 48 wherein several image instances aresuccessively generated by modifying parameters of the parametrizedtransformation, said set of image instances forming a displayable imageanimation.
 50. Method according to claim 49 wherein said parameters aremodified smoothly as a function of time to yield a smoothly evolvinganimated microstructure.
 51. Method according to claim 41 wherein therendering of the microstructure and original image includes a standardor multicolor dithering operation.
 52. Method according to claim 41further including applying a mask specifying a region of the originalimage to be rendered with the microstructure.
 53. Method according toclaim 41 further including applying a multi-valued mask expressingweights of original image colors and weights of the selected basiccolors for generating the image.
 54. Method according to claim 41wherein the microstructure includes information personal to a user ofthe image.
 55. Method according to claim 41 wherein the microstructureincludes information identifying and specific to a particular event ortransaction, such as date, venue, seating, destination, time.
 56. Methodaccording to claim 41 wherein the microstructure elements includealphanumerical characters provided at a size in relation to the imagethat allow their reading at a personal document reading distance. 57.Method according to claim 42 wherein the high-frequency microstructureelements are placed at locations corresponding to the background of thelow frequency microstructure elements.
 58. Method according to claim 57wherein the comparison yields a deltamap which is dithered by a highfrequency dither array, the resulting dithered deltamap being composedwith the dithered image.
 59. Method according to claim 41 wherein a maskwhose shape expresses a visual message specifies the part of themicrostructure that is to be printed with special inks that enable,under certain observation conditions, the mask shape to remain hiddenwithin the image and under other observation conditions, the mask shapeto be clearly revealed.
 60. Method according to claim 59 wherein partsof the image specified by the mask are printed with a special inkselected from a group of metallic and iridescent inks, whereby the maskshape is hidden at a certain observation angle and is visible at adifferent observation angle.
 61. Method according to claim 60 whereinparts of the the image specified by the mask are printed with a specialink invisible in daylight and visible in light at selected frequencies,such as Ultraviolet light.
 62. Method according to claim 41 furtherincluding defining color information used for rendering the targetimage; defining parameters of a parametrized transformation; traversinga target image positions (x,y) pixel by pixel and row by row,determining corresponding positions in the original image (x′,y′) and,according to the parametrized transformation, corresponding positions inthe microstructure (x″,y″); obtaining from the original image positions(x′,y′) the color C_(r) to be reproduced and from the microstructurepositions (x″,y″) rendering information; rendering the target image bymaking use of the rendering information.
 63. Method of generating asecurity document comprising producing an image incorporating amicrostructure according to a method set forth in claim 41, wherein theoriginal image rendered with the microstructure comprises informationrelevant to the purpose of the document and intended to be protectedagainst counterfeiting.
 64. Method of generating a security document forprinting or display, including the steps of: selecting or retrieving anoriginal image; selecting or retrieving information specific to aperson, an event or transaction to which said security document relates;generating a microstructure comprising readable microstructure elementsproviding information on said person, event or transaction; renderingsaid original image with said microstructure image using the methodaccording to claim
 41. 65. Method according to claim 64 wherein theimage is printed or displayed on a support selected from a groupincluding any of paper, plastics, polymers, product packages, opticaldisks, and optical devices comprising holograms, kinegrams anddiffractive elements.
 66. Method according to claim 64 wherein thesecurity document is a commercial instrument bearing value or relatingto a commercial transaction.
 67. Method according to claim 64 whereinthe security document is an certificate, title or deed.
 68. Methodaccording to claim 64 wherein the security document includes informationidentifying a person or entity.
 69. Method according to claim 64,wherein the image incorporating a microstructure is generated in aserver system, said server system preparing and packaging a displayablefile for printing said security document on a standard printer or fordisplay on an electronic screen, such as a screen of a portableelectronic device.
 70. Method according to claim 69, wherein saidsecurity document is printed on said standard printer at a customer siteremote from said server system and accessible to said server system viaa communications network such as the internet.
 71. Method according toclaim 64 wherein some or all information included in the original imageis retrieved from one or more databases via a global communicationsnetwork such as the internet.
 72. Method according to claim 64 whereinthe original image comprises a portrait of a bearer selected from acustomer database on the basis of information identifying said bearer.73. An image incorporating a microstructure generated by a methodaccording to claim 41 wherein the microstructure elements compriseinformation specific to a particular event, transaction, or person. 74.A security document comprising an image according to claim
 73. 75. Acomputing system for synthesizing a security document, comprising: aninterface operable for receiving a request for synthesizing the securitydocument, a preparation software module operable for preparing datafiles according to document related information received with therequest, where the preparation of data files comprises the generation ofan original document image, the generation of microstructure shapes andthe generation of transformation parameters, and a production softwaremodule operable for producing the security document, where producing thesecurity document comprises the synthesis of a microstructure and thesynthesis of a security document with that microstructure.
 76. Computingsystem according to claim 75, wherein microstructure shapes aregenerated by producing a bitmap incorporating the microstructure shapes,where the microstructure is embodied in a dither array synthesized fromsaid bitmap by applying to it mathematical morphology operations andwhere the security document is synthesized by dithering the originaldocument image with the synthesized dither array.